default_charset = nil produces bogus content-type header

To work around bugs in IE7, it is necessary to sometimes not send a charset parameter in a content-type header. See http://golem.ph.utexas.edu/~dist... for an example.

Setting ActionController.default_charset = nil would seem to address this, but unfortunately it results in a header such as the following to be sent:

Content-Type: application/xhtml+xml; charset=

The suggested fix is to make ActionController::AbstractResponse#charset= match the behavior of content_type= and not insert the charset parameter into the content type if nil.

An alternate fix (one that would be less failsafe) would be to modify AbstractResponse#assign_default_content_type_and_charset to not set response.charset if the default is nil.