Multipart upload fixes

Had issues with the library when attempting to perform file uploads via various services.

The first commit for `CurlClient` fixes the issue whereby request data was manually encoded to a querystring before being sent through to cURL. This prevents multipart transfers loading since cURL requires arrays in this case. I don't believe this step should be necessary at all with cURL as it does it automatically for normal form-url-encoded requests but will leave it in just in case. I haven't made a fix for the stream client but I have had many other issues with that transport anyway.

The second commit fixes an issue which I know specifically affects twitter (see #242), but probably applies to other OAuth1 services as well. Multipart uploads only have the `oauth_` parameters in the authorization header hashed, everything else is not included in that verification since (I presume) the size of the binary post data would often be rather enormous. There is more info on this here: https://dev.twitter.com/docs/uploading-media