Ensure consistent "Accept" Ajax header across browsers

Most browsers send `*/*` by default, but Firefox sets:

  text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8

This change always sets the default header to `*/*` in all browsers to
ensure consistent server responses and make our Ajax tests pass that
were failing for Firefox on "Accept" header verification.

Fixes #867