Write form data into request body before signing/sending.

In support of https://github.com/awssum/awssum-amazon-cloudformation/pull/1

This ensures that any arguments declared as form-data are actually sent along with request.  Encoding that at this point ensures that the body is prepared before the request signature is generated.

I've no idea whether the proposed "form-data" argument type will work, I'll continue playing with it an update if I run into any errors.