Fix for grails 2.3.10

In Grails 2.3.10, RestClient requests for xml documents with a doctype declaration in the markup fail with the following error:

DOCTYPE is disallowed when the feature "http://apache.org/xml/features/disallow-doctype-decl" set to true.

Grails 2.3.0 apps do not throw this error while parsing the same markup.  It looks like the default settings for XmlSlurper may have changed between groovy 2.1.6 (ships with grails 2.3.0) and groovy 2.1.9 (ships with grails 2.3.10).

This change modifies ResponseBuilder to create its own SAXParser and pass it into the XmlSlurper.  The SAXParser is built with identical settings to the one XmlSlurper would build in groovy 1.7.6 (when the XmlSluper object is created with the no argument constructor). Basically it forces the parser to use the 1.76 defaults in cases where we are not able to ship with that version of groovy.

If this isn't the right fix let me know and I'm happy to work with you on implementing the right one.

Thanks.