Update ServiceClientBase.cs

Changed signature of PostFileWithRequest so that it reports progress through the ProgressDelegate, which can be either provided as a parameter (like in this commit) or as a property like it's already the case with OnDownloadProgress.
Sorry if this doesn't compile I've edited it online in GitHub's editor ;)
This is only to illustrate our feature request from here:
http://servicestack.uservoice.com/forums/176786-feature-requests/suggestions/5374611-add-an-overload-of-postfilewithrequest-which-accep
Unless I'm missing something obvious this is really a simple change that shouldn't take long, but I've been incorrect before :) so please let me know if there are other concerns our if you need additional info, thanks!