Replace `font-lock-fontify-buffer` with font-lock-ensure and resolve free variable issue

As the documentation for `font-lock-fontify-buffer` says:

    When called from Lisp, this function is a big mess.  The caller usually
    expects one of the following behaviors:
    - refresh the highlighting (because the font-lock-keywords have been
      changed).
    - apply font-lock highlighting even if font-lock-mode is not enabled.
    - reset the highlighting rules because font-lock-defaults
      has been changed (and then rehighlight everything).
    Of course, this function doesn't do all of the above in all situations
    (e.g. depending on whether jit-lock is in use) and it can't guess what
    the caller wants.

In `restclient-replace-all-in-headers` replace the free variable `vars`
with `replacements`.