code: def commit(self, registerID, checksum=None):
          
          parts = ",".join(self.parts(registerID=registerID))
          url = self._url + "/%s/commit"
          params = {
              "f" : "json",
              "parts" : parts
          }
          if checksum is not None:
              params['checksum'] = checksum
          return self._post(url=url,
                               param_dict=params,
                               securityHandler=self._securityHandler,
                               proxy_url=self._proxy_url,
                               proxy_port=self._proxy_port)
docs: Once a multipart upload is completed, the files need to be merged
          together.  The commit() does just that.
          Inputs:
           registerID - unique identifier of the registered item.
           checksum - upload id.
          output:
           dictionary