Decodable response serialization and major refactor (#2265)

* First draft of response serializer refactor and decodable serializers.

* Refactor serializer protocols and implementations.

* Finish refactors, update inline docs.

* Remove download serializer from Data, as it’s default now.

* Update whitespace.

* Add failure expectation test.

* Remove unnecessary assert.

* Refactor serializers to use throws instead of Result.

Source/ResponseSerialization.swift

@@ -56,14 +56,14 @@ public extension DownloadResponseSerializerProtocol where Self: DataResponseSeri
         guard let fileURL = fileURL else {
             throw AFError.responseSerializationFailed(reason: .inputFileNil)
         }
-
+        
         let data: Data
         do {
             data = try Data(contentsOf: fileURL)
         } catch {
             throw AFError.responseSerializationFailed(reason: .inputFileReadFailed(at: fileURL))
         }
-
+        
         do {
             return try serialize(request: request, response: response, data: data, error: error)
         } catch {
@@ -392,7 +392,7 @@ public final class StringResponseSerializer: ResponseSerializer {
         }
 
         let actualEncoding = convertedEncoding ?? .isoLatin1
-
+        
         guard let string = String(data: validData, encoding: actualEncoding) else {
             throw AFError.responseSerializationFailed(reason: .stringSerializationFailed(encoding: actualEncoding))
         }

Tests/ResponseSerializationTests.swift

@@ -433,7 +433,7 @@ class DataResponseSerializationTestCase: BaseTestCase {
     }
 
     // MARK: JSONDecodableResponseSerializer
-
+    
     struct DecodableValue: Codable {
         let string: String
     }