code: private JsonClassWriterBase getCustomWriter(Class c)
      {
          JsonClassWriterBase writer = writerCache.get(c);
          if (writer == null)
          {
              writer = forceGetCustomWriter(c);
              writerCache.put(c, writer);
          }
          return writer == nullWriter ? null : writer;
      }
docs: Fetch the customer writer for the passed in Class.  If it is cached (already associated to the
  passed in Class), return the same instance, otherwise, make a call to get the custom writer
  and store that result.
  @param c Class of object for which fetch a custom writer
  @return JsonClassWriter/JsonClassWriterEx for the custom class (if one exists), null otherwise.