Only set request content type for create/update

backbone.js

@@ -971,7 +971,6 @@
     // Default JSON-request options.
     var params = _.extend({
       type:         type,
-      contentType:  'application/json',
       dataType:     'json',
       processData:  false
     }, options);
@@ -983,6 +982,7 @@
 
     // Ensure that we have the appropriate request data.
     if (!params.data && model && (method == 'create' || method == 'update')) {
+      params.contentType = 'application/json',
       params.data = JSON.stringify(model.toJSON());
     }