Allow to force json

In case that upstream api doesn't follow the HTTP spec. So we can
force to parse response body as json; vice versa.

Signed-off-by: Clark Wang <clark.wangs@gmail.com>
Add form support to request builder

Signed-off-by: Clark Wang <clark.wangs@gmail.com>
improve property existence test

Signed-off-by: Clark Wang <clark.wangs@gmail.com>
Merge branch 'feature/form'

Conflicts:
	lib/rest-builder.js