Add better support for adding form file parameters to web requests

[bdach]

Something I ended up needing in the course of BSS work.

The previous implementation would treat files as a straight dictionary, as in you could have only one file with a given parameter name in a request. This isn't how multipart/form-data really works; you're allowed to send multiple files in scope of a single parameter.

Additionally, the request did not permit specifying a filename, which is yet another thing I need for BSS purposes.

I don't believe this method has any real consumers, yet anyhow:

Breaking changes

WebRequest.AddFile() has been altered to provide an API closer to what the HTTP specs intended, and to allow sending multiple files for a given request parameter. To that end:

• The signature of the method has changed from

  public void AddFile(string name, byte[] data);

  to

  public void AddFile(string paramName, byte[] data, string filename = "blob");

• Calling .AddFile() with the same first parameter (formerly called name, now called paramName) twice will result in two files being sent in the request, rather than the second call overwriting the file added by the first one.

• If the filename parameter is not explicitly specified in the method call, the filename given in the Content-Disposition header of the multipart request has changed from being equal to the parameter name to blob. This is consistent with JavaScript FormData API behaviour.