Add support for custom transformAndCheckStatus transform, and forcing json transforms

[aaronc-kablamo]

Adds a new optional responseTransform parameter to the original transformAndCheckStatus, and adds transformJsonAndCheckStatus as a standalone convenience function which always forces the response.json() response even when the Content-Type isn't supplied or doesn't match

[coveralls]

coverage: 78.971% (-0.3%) from 79.279%
when pulling 4f8ecf6 on feature/transformJsonAndCheckStatus
into 1c55f18 on master.