Form 'action' fix for non-canonical document roots

The 'action' attribute on jFormer forms is correctly determined when
the website's document root is a non-canonical path, e.g. a symlink.

NOTE: This will not fix path problems that people might encounter if the form-processing PHP file itself uses a non-canonical path that is not a sub-path of the document root. In that case it would probably be better to pass $_SERVER['PHP_SELF'] as the 'action' option when constructing the JFormer object.