Edited Issue: Default buffer size in MultipartFileStreamProvider should be 32k [369]

The default *WRITE* buffer size in MultipartFileStreamProvider should match the value of the default *READ* buffer size in HttpContentMultipartExtensions.ReadAsMultipartAsync. Using the same buffer size for both reading and writing would result in less overhead. The current default READ buffer size is 32k but the default WRITE buffer size is 4k.