Document that a Content-Type header is required

- I was caught out by not specifying this header and the app 503ing.
Merge pull request #40 from alphagov/document_content_type_header_as_required

Document that a Content-Type header is required
Return the GOV.UK URL to the content in the success JSON response

- Put the `govuk_url` method onto the `publishing_api_[manual|section]`
  models to avoid mixing internal publishing up with external manual
  validation.
- Memoize (make the variable start with `@_` (?)) to avoid calling the
  `publishing_api_manual` or `publishing_api_section` methods thrice in
  each controller.

(Thanks to Jenny for helping with this.)
Send a Location header on a success response, as well as the body

- The tests for the presence of `/guidance/MANUAL_SLUG` come after the
  assertion `assert_publishing_api_put_item`, because it made more sense
  to me to have them after the thing is sent to publishing API, even
  though the test passes with it before that assertion.