Encode request data as JSON when Content-Type is set to application/json

[danhardman]

The issue has been open for a while with a bunch of +1s so I thought I'd give it a go.

This is untested as npm test wasn't able to run, something to do with spawning phantomjs so if anyone could give me a hand figuring out how to test this properly that'd be great. However, it's a pretty simple change so everything should be good.

[AWare]

I got caught out by this issue today, this pr would be really helpful.

[danhardman]

@AWare This repo is dead, I recommend finding an alternative!