A few changes related to XFO, a typo fix, and spec test description improvements

* Two changes to X-Frame-Options, as per current spec draft
  (https://tools.ietf.org/html/draft-ietf-websec-x-frame-options-02)
  - Use 'X-Frame-Options' instead of 'X-FRAME-OPTIONS'
  - Make the colon after X-Frame-Options: ALLOW-FROM optional

* Fix typo in README for 'widely supported' config

* Improve spec test descriptions