controller: default to JSON for REST API

If no request format is specified for an REST API call, default to
responding with JSON.

This commit fixes bug 1322543.

https://bugzilla.redhat.com/show_bug.cgi?id=1322543

controller/lib/openshift/controller/api_behavior.rb

@@ -8,11 +8,16 @@ module ApiBehavior
 
       included do
         before_filter ->{ Mongoid.identity_map_enabled = true }
+        before_filter :default_format_json
       end
 
       protected
         attr :requested_api_version
 
+        def default_format_json
+          request.format ||= 'json'
+        end
+
         def check_version
           version = catch(:version) do
             "#{request.accept},#{request.env['CONTENT_TYPE']}".split(',').each do |mime_type|