Execute an update expression.  Should generally only be called implicitly.

def execute_update(self, update, safe=False):
		''' Execute an update expression.  Should generally only be called implicitly.
		'''

		# safe = self.safe
		# if update.safe is not None:
		#     safe = remove.safe

		assert len(update.update_data) > 0
		self.queue.append(UpdateOp(self.transaction_id, self, update.query.type, safe, update))
		if self.autoflush:
			return self.flush()