Implement MKNetworkOperation copyForRetry.

This copies the MKNetworkOperation, with the intention of retrying the call.
This means that the request parameters and callbacks are all preserved, but
anything related to an ongoing request is discarded, so that a new request
with the same configuration can be made.
Implement MKNetworkOperation addHeader.

This adds one single header, but otherwise acts like addHeaders.
Merge branch 'copyForRetry'
Merge branch 'addHeader'