New Assertions type: FileResponseAssertions

[KPull]

Similarly to #24, we need to add a new type of assertions object, FileResponseAssertions, that will load the binary data from the given file or resource and compare it to the content body received from the server.

The FileResponseAssertions should try to guess what the Content-Type header should be depending on the file chosen (some files will even have a MIME type embedded within them).