Add support for spaces.

Prior to this commit, spaces caused a redirect on our Ajax POSTs,
turning them into GETs. This confused the callback, making it think
the call was non-ajax. This patch adds support for checking whether a
path will be rewritten by spaces. If so, we seek to rewrite the callback
to the 'correct' version so the POST can continue.

Signed-off-by: Nigel Cunningham <nigel@nigelcunningham.com.au>