Added fieldName property to PostFileWithRequest

I have added optional `fieldName` property to `PostFileWithRequest<TResponse>` method. As noted in [this SO question](http://stackoverflow.com/q/18326403/1295344) the field name to which the upload is made, is currently hardcoded to the value *"upload"*, and this limits the usability of the method.

This optional parameter allows the field name to be specified, and if not specified it defaults to the original value "upload", so this addition will not break any existing functionality.