Update progress when using multipart requests

There is a problem with upload progress not being updated when making multipart uploads. Multipart requests built using ``` AFStreamingMultipartFormData  ``` do not report any progress because ``` NSURLSessionUploadTask ``` relays on requests Content-Length header. But, at the same time that header is removed when ``` NSURLSessionUploadTask ``` is being constructed. So no upload progress can be reported. 
Please refer to issue #1398 for detailed discussion on what's wrong. 

This can be solved because ``` AFStreamingMultipartFormData ``` correctly calculates content-length.

Not sure if this can be merged, but it's quite helpful when you want to upload image with extra post data.