When the reply body is JSON.stringified, it also sets the content-type header to application/json.
previously only addressed the JSON encoding for reply callbacks. Now we're covered.
Merge branch 'json_content_type' of github.com:spenceralger/nock into spenceralger-json_content_type

Conflicts:
	lib/request_overrider.js
minor version bump to 0.27.0