Interceptor should only work on String response

[janblok]

When using:
$httpProvider.interceptors.push('xmlHttpInterceptor');
it tries to convert the response even when it not a xml string anymore (and fails)

Example scenario, in the application an XML is received and transformed like:
transformResponse : function(data) {
return $.parseXML(data);
}
to a dom tree, to be used for xpath expressions.
In this scenario its not required for x2js to kick in and try to convert to json, since its not an xml string anymore