Use LoggerAPI debug logging instead of print in DataSender (#140)

This gives users of Swift-SMTP more control over stdout output.

.travis.yml

@@ -11,21 +11,21 @@ branches:
 # by .swift-version, unless SWIFT_SNAPSHOT is specified.
 matrix:
   include:
-    - os: linux
-      dist: xenial
-      sudo: required
-      services: docker
-      env: DOCKER_IMAGE=docker.kitura.net/kitura/swift-ci-ubuntu16.04:5.2.5 DOCKER_ENVIRONMENT="EMAIL PASSWORD"
     - os: linux
       dist: bionic
       sudo: required
       services: docker
-      env: DOCKER_IMAGE=docker.kitura.net/kitura/swift-ci-ubuntu18.04:5.4 DOCKER_ENVIRONMENT="EMAIL PASSWORD"
+      env: DOCKER_IMAGE=docker.kitura.net/kitura/swift-ci-ubuntu18.04:5.2.5 DOCKER_ENVIRONMENT="EMAIL PASSWORD"
     - os: linux
-      dist: xenial
+      dist: focal
       sudo: required
       services: docker
-      env: DOCKER_IMAGE=docker.kitura.net/kitura/swift-ci-ubuntu18.04:latest SWIFT_SNAPSHOT=$SWIFT_DEVELOPMENT_SNAPSHOT DOCKER_ENVIRONMENT="EMAIL PASSWORD"
+      env: DOCKER_IMAGE=docker.kitura.net/kitura/swift-ci-ubuntu20.04:5.4 DOCKER_ENVIRONMENT="EMAIL PASSWORD"
+    - os: linux
+      dist: focal
+      sudo: required
+      services: docker
+      env: DOCKER_IMAGE=docker.kitura.net/kitura/swift-ci-ubuntu20.04:latest USE_SWIFT_DEVELOPMENT_SNAPSHOT=1 DOCKER_ENVIRONMENT="EMAIL PASSWORD"
     - os: osx
       osx_image: xcode12.2
       sudo: required

Sources/SwiftSMTP/DataSender.swift

@@ -15,6 +15,7 @@
  **/
 
 import Foundation
+import LoggerAPI
 
 // Used to send the content of an email--headers, text, and attachments.
 // Should only be invoked after sending the `DATA` command to the server.
@@ -200,13 +201,13 @@ extension DataSender {
 private extension DataSender {
     // Write `text` to the socket.
     func send(_ text: String) throws {
-        print("SEND: \(text)")
+        Log.debug("SEND: \(text)")
         try socket.write(text)
     }
 
     // Write `data` to the socket.
     func send(_ data: Data) throws {
-        print("SEND: data \(data.count) bytes")
+        Log.debug("SEND: data \(data.count) bytes")
         try socket.write(data)
     }
 }