feat(heartbeat resource): cache json response body

Author: bethesque

lib/pact_broker/diagnostic/resources/heartbeat.rb

@@ -14,7 +14,7 @@ def content_types_provided
         end
 
         def to_json
-          {
+          @@json ||= {
             "ok" => true,
             "_links" => {
               "self" => {
@@ -23,7 +23,6 @@ def to_json
             }
           }.to_json
         end
-
       end
     end
   end