Add a :mashify option to turn off the Mashify middleware

We have apps that use different libraries for talking to Salesforce. We have some code that is shared for doing things with the responses, so we need the responses in the same format. The simplest way to guarantee that is to use the response as returned by Salesforce. This change makes it easier to disable the Mashify middleware when we need to for Restforce.

This option makes it possible to get raw responses without directly modifying the middleware stack.
    
The Mashify middleware is not inserted if the :mashify option == false. The default is nil to retain back compatibility.
    
The Restforce::Concerns::Connection#mashify? method is unmodified. It still looks at the middleware stack to determine if it includes Mashify, so there should be no change in behavior for existing uses that modify the
stack to remove Mashify.