[#1406] URL Encode the path parameter for render_exception

Fixes #1406

If a request is made and path is something like `/%d3` we rescue this with
a custom 404 template.

This gets unescaped as `{"path"=>"\323"}`.

In the case of a `RouteNotFound`, `ApplicationController#render_exception`
renders the `general/exception_caught` template in to the default layout,
which renders the `general/_locale_switcher` partial.

This partial calls `url_for` – sending the full params hash as the
argument – so that a user may return to the existing page in their
chosen locale.

The problem is that url_for tries to construct the url with the hash
`{:action=>"not_found", :controller=>"general",
:path=>"\323"}`.

`ApplicationController#sanitize_params` re-encodes the path parameter so
that it can be passed through to url_for without trouble.