Fix exception when sending actual multipart request (Fix #250)

Author: konomae

Sources/APIKit/BodyParameters/MultipartFormDataBodyParameters.swift

@@ -324,5 +324,13 @@ public extension MultipartFormDataBodyParameters {
         override func remove(from aRunLoop: RunLoop, forMode mode: RunLoopMode) {
 
         }
+
+        override func property(forKey key: Stream.PropertyKey) -> Any? {
+            return nil
+        }
+
+        override func setProperty(_ property: Any?, forKey key: Stream.PropertyKey) -> Bool {
+            return false
+        }
     }
 }