[PATCH] file_field_tag makes the enclosing form multipart (see b17b980).

A great feature was added in b17b980a15ee4a5f2f629d91c6fa6ca7db74daa8 which sets form_for to multipart automatically. This patch adds that feature to form_tag as well.

This is a first implementation using String#include? to check the captured block.

Consider this ugly, but a good working implementation. A better approach would probably be an inner class to build the form, but that's quite an undertaking and I wanted to float the idea here first.