Fix requests-mock reqcallable with FileObject

Some refactoring and additional unit testing provided by #78 exposed an issue (see https://github.com/TestInABox/stackInABox/pull/78/files#r553752832) that was present prior but just now exposed by the unit test. Not sure where the issue is as the `request-mock` code is relying on `urllib3` and `requests` code to perform some functionality.