Use HTTP POST instead of PUT for assetStatusUpdateUrl

[quicksketch]

We recently ran into an issue when using this library that our assetStatusUpdateUrl URLs were returning HTTP 501 (not supported) errors. This turned out to be because this library uses a "PUT" request when the upload is finiahed, and our callback URL was hosted through Akamai, which by default does not allow PUT requests.

Really this shouldn't be a PUT request in the first place, since this URL is called after the file has been finished uploading. It's not really "putting" anything on our server, it's just notifying the server that the upload is complete.

I'd like to propose that we just change this PUT request to POST, to prevent these sort of problems, and to use the proper request type for the situation.

For reference: http://stackoverflow.com/questions/107390/whats-the-difference-between-a-post-and-a-put-http-request