Add the ability to set the GuzzleHttp\Client instance on the PHRETS\Session

[collegeman]

We use PHRETS to connect to and download data from MRIS/BrightMLS.

We have a use case that requires our application, in a single runtime, to connect to more than one server. As is, this is impossible because the PHRETS\Session calls PHRETSClient to manage a singleton instance of GuzzleHttp\Client. This makes it impossible to authenticate more than one RETS session per runtime, which we need to be able to do.

This is solved by adding a mutator to PHRETS\Session, allowing for our application to set the instance of GuzzleHttp\Client at the time the session object is created.

All of your unit tests are passing.