Fallback to non-skeleton Header (#416)

Sometimes my collectionview's header doesn't need to be 'skeletoned' while the content does.  

When  collectionSkeletonView(_ skeletonView: UICollectionView, supplementaryViewIdentifierOfKind: String, at indexPath: IndexPath) -> ReusableCellIdentifier? returns nil, it's best if we fallback to the non-skeleton header.

README.md

@@ -4,10 +4,12 @@
     <a href="https://github.com/Juanpe/SkeletonView/actions?query=workflow%3ACI">
       <img src="https://github.com/Juanpe/SkeletonView/workflows/CI/badge.svg">
     </a>
-    <a href="https://codebeat.co/projects/github-com-juanpe-skeletonview-master"><img alt="codebeat badge" src="https://codebeat.co/badges/f854fdfd-31e5-4689-ba04-075d83653e60" /></a>
-    <img src="http://img.shields.io/badge/dependency%20manager-swiftpm%2Bcocoapods%2Bcarthage-green" />
-    <img src="https://img.shields.io/badge/platforms-ios%2Btvos-green" />
-    <a href="https://badge.bow-swift.io/recipe?name=SkeletonView&description=An%20elegant%20way%20to%20show%20users%20that%20something%20is%20happening%20and%20also%20prepare%20them%20to%20which%20contents%20he%20is%20waiting&url=https://github.com/juanpe/skeletonview&owner=Juanpe&avatar=https://avatars0.githubusercontent.com/u/1409041?v=4&tag=1.8.7"><img src="https://raw.githubusercontent.com/bow-swift/bow-art/master/badges/nef-playgrounds-badge.svg" alt="SkeletonView Playground" style="height:20px"></a>   
+    <a href="https://codebeat.co/projects/github-com-juanpe-skeletonview-main"><img alt="codebeat badge" src="https://codebeat.co/badges/1f37bbab-a1c8-4a4a-94d7-f21740d461e9" /></a>
+    <a href="https://cocoapods.org/pods/SkeletonView"><img src="https://img.shields.io/cocoapods/v/SkeletonView.svg?style=flat"></a>
+    <a href="https://github.com/Carthage/Carthage/"><img src="https://img.shields.io/badge/Carthage-compatible-4BC51D.svg?style=flat"></a>
+    <a href="https://swift.org/package-manager/"><img src="https://img.shields.io/badge/SPM-supported-Green.svg?style=flat"></a>
+    <img src="https://img.shields.io/badge/platforms-iOS_tvOS-green" />
+    <a href="https://badge.bow-swift.io/recipe?name=SkeletonView&description=An%20elegant%20way%20to%20show%20users%20that%20something%20is%20happening%20and%20also%20prepare%20them%20to%20which%20contents%20he%20is%20waiting&url=https://github.com/juanpe/skeletonview&owner=Juanpe&avatar=https://avatars0.githubusercontent.com/u/1409041?v=4&tag=1.20.0"><img src="https://raw.githubusercontent.com/bow-swift/bow-art/master/badges/nef-playgrounds-badge.svg" alt="SkeletonView Playground" style="height:20px"></a>   
 </p>
 
 <p align="center">
@@ -43,6 +45,7 @@ Enjoy it! 🙂
 - [✨ Miscellaneous](#-miscellaneous)
 - [❤️ Contributing](#️-contributing)
 - [📢 Mentions](#-mentions)
+- [🏆 Sponsors](#-sponsors)
 - [👨🏻‍💻 Author](#-author)
 - [👮🏻 License](#-license)
 
@@ -563,10 +566,7 @@ Then, when the skeleton appears, you can see the view hierarchy in the Xcode con
 
 * iOS 9.0+
 * tvOS 9.0+
-* Swift 5
-
-## :sweat_smile: Need help
-If you need help about how to use **SkeletonView**, use [Stack Overflow](https://stackoverflow.com/questions/tagged/skeletonview) and tag `skeletonview`.
+* Swift 5.3
 
 ## ❤️ Contributing
 This is an open source project, so feel free to contribute. How?
@@ -596,7 +596,12 @@ For more information, please read the [contributing guidelines](https://github.c
 - [Swift News #36](https://www.youtube.com/watch?v=mAGpsQiy6so)
 - [Best iOS articles, new tools & more](https://medium.com/flawless-app-stories/best-ios-articles-new-tools-more-fcbe673e10d)
 
+## 🏆 Sponsors
 
+Open-source projects cannot live long without your help. If you find **SkeletonView** is useful, please consider supporting this 
+project by becoming a sponsor. 
+
+Become a sponsor through [GitHub Sponsors](https://github.com/sponsors/Juanpe) :heart:
 
 ## 👨🏻‍💻 Author
 

SkeletonView.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name         = "SkeletonView"
-  s.version      = "1.19.0"
+  s.version      = "1.20.0"
   s.summary      = "An elegant way to show users that something is happening and also prepare them to which contents he is waiting"
   s.description  = <<-DESC
   Today almost all apps have async processes, as API requests, long runing processes, etc. And while the processes are working, usually developers place a loading view to show users that something is going on.

Sources/Collections/SkeletonCollectionDataSource.swift

@@ -107,7 +107,7 @@ extension SkeletonCollectionDataSource: UICollectionViewDataSource {
             return view
         }
         
-        return UICollectionReusableView()
+        return originalCollectionViewDataSource?.collectionView?(collectionView, viewForSupplementaryElementOfKind: kind, at: indexPath) ?? UICollectionReusableView()
     }
     
 }