Merge pull request Alamofire#254 from mowens/bug/handle-0-bytes

Added hanlding of valid NSData object with 0 bytes for serializers

Author: mattt

Source/Alamofire.swift

@@ -1366,7 +1366,7 @@ extension Request {
     */
     public class func JSONResponseSerializer(options: NSJSONReadingOptions = .AllowFragments) -> Serializer {
         return { (request, response, data) in
-            if data == nil {
+            if data == nil || data?.length == 0 {
                 return (nil, nil)
             }
 
@@ -1415,7 +1415,7 @@ extension Request {
     */
     public class func propertyListResponseSerializer(options: NSPropertyListReadOptions = 0) -> Serializer {
         return { (request, response, data) in
-            if data == nil {
+            if data == nil || data?.length == 0 {
                 return (nil, nil)
             }