Convert data-association-insertion-template to string before escaping to...

If CGI has been monkeypatched by the https://github.com/brianmario/escape_utils gem, we need to convert the rendered association of link_to_add_association from ActiveSupport::SafeBuffer to String in order for it to be escaped. 
Otherwise, the links will look like this:

![screenshot from 2014-01-21 16 30 50](https://f.cloud.github.com/assets/61476/1961745/e9fabdcc-8279-11e3-9188-05afbb76263b.png)

Please see https://github.com/gsmendoza/cocoon_simple_form_demo/tree/cgi-escape-html-active-support-safe-buffer-bug for a demo of the bug and its fix.