Provide a variable for default response format in rest_server

It would be nice to be able to have a user-defined fallback (i.e. variable_get/set). This was possible in 7.x-3.3 and earlier before the major architectural changes, but ServicesContentTypeNegotiator class is in charge of this now. In order to change this I would need to rewrite rest_server or hack services.

ServicesContentTypeNegotiator::getResponseFormatContentTypeNegotiations()

diff --git a/servers/rest_server/includes/ServicesContentTypeNegotiator.inc b/servers/rest_server/includes/ServicesContentTypeNegotiator.inc
index cc7ec27..4b2f1fb 100644
--- a/servers/rest_server/includes/ServicesContentTypeNegotiator.inc
+++ b/servers/rest_server/includes/ServicesContentTypeNegotiator.inc
@@ -84,7 +84,7 @@ class ServicesContentTypeNegotiator implements ServicesContentTypeNegotiatorInte
     }
 
     // Get the best matching format, default to json
-    $response_format = 'json';
+    $response_format = variable_get('rest_server_default_response_format', 'json');
     $http_accept = $context->getServerVariable('HTTP_ACCEPT');
     if (!empty($http_accept)) {
       $mime = $this->mimeParse();

I will post a patch shortly.