Add isEncryptionMethod property to 7zCoder and use it in 7zFolder

Sources/7-Zip/7zCoder.swift

@@ -13,6 +13,7 @@ class SevenZipCoder {
 
     let id: [UInt8]
     let compressionMethod: CompressionMethod
+    let isEncryptionMethod: Bool
 
     let numInStreams: Int
     let numOutStreams: Int
@@ -31,6 +32,7 @@ class SevenZipCoder {
         let id = bitReader.bytes(count: idSize)
         self.id = id
         compressionMethod = CompressionMethod(id)
+        isEncryptionMethod = id[0] == 0x06
 
         numInStreams = isComplex ? bitReader.szMbd() : 1
         numOutStreams = isComplex ? bitReader.szMbd() : 1

Sources/7-Zip/7zFolder.swift

@@ -190,7 +190,7 @@ class SevenZipFolder {
                 try decoder.decode()
                 decodedData = Data(bytes: decoder.out)
             default:
-                if coder.id[0] == 0x06 {
+                if coder.isEncryptionMethod {
                     throw SevenZipError.encryptionNotSupported
                 } else {
                     throw SevenZipError.compressionNotSupported