v1.1.7: Agreement Form Data Content Type

* A request for the agreement form data requires a content type to be
set even if it doesn't match our request. This matches the behavior of
the swagger api docs.

v1.1.6: Transient Document Upload

* Fix the format of the transient document upload so it can be sent as a
guzzle 6 mulipart request.
* Simplify the use of the guzzle client to send the request.
* Don't double encode the json as this causes problems.

v1.1.5: Set empty body to null

* The value of the body coming from the httpRequest is an array which is not
properly handled.

v1.1.4: Fixing JSON handling

* Typo fix for encoding json requests and fixing json decoding of the response
due to the removal of the json method from PSR7 responses.

v1.1.3: Bug fix for saving files

* Need to pass the sink option to the guzzle send method to get it to properly
save the file.

v1.1.2: Fix handling of the request body

* The new PSR7 request doens't use an arry of options for the body, so we need
to pass a string body into the Request constructor

v1.1.1: Use PSR-7 Request Object

* Update the GuzzleTransport to use PSR-7 Request Objects so it is compatible
with Guzzle 6. This breaks backwards compatibility with Guzzle 5