Factor out wrapping response text in textarea
Factor out wrapping response text in textarea

The jQuery Iframe Transport can't access the status code or content type
of the request targeted to the iframe. It provides a mechanism to fake
this by wrapping the response body in a textarea element with various
properties set as data attributes on the element.

I use this to wrap the JSON output and broke out the code that generates
the textarea output into a separate method anticipating using this in
other cases, such as wrapping error responses.

Addresses #595

denverfoundation/storybase,denverfoundation/storybase,denverfoundation/storybase,denverfoundation/storybase