[PR #2705] Allow changing boundary string in upload:multipartFormData callback

Source/MultipartFormData.swift

@@ -98,7 +98,7 @@ open class MultipartFormData {
     public var contentLength: UInt64 { return bodyParts.reduce(0) { $0 + $1.bodyContentLength } }
 
     /// The boundary used to separate the body parts in the encoded form data.
-    public let boundary: String
+    public var boundary: String
 
     private var bodyParts: [BodyPart]
     private var bodyPartError: AFError?