Add missing response key migration for `html -> body`.

(This was missed in 5186b2b2a8e80cfc05d70420810f082e35442e0e.)

Add tests to ensure response processing for html applies to both the
new `body` field and the deprecated `html` field.

Progress on #24.
Merge pull request #36 from nicksay/migrate-keys

Add missing response key migration for `html -> body`.