Feature request: Making FieldTables iterateable

Hi, first off, I love the lib. I'm working a lot with message headers and it's a bit hard at the moment to work with amqprs' FieldTable type. I might have missed something, but currently I don't see how I can iterate through the header key-values. This feature would come in handy, when I don't know all the header keys and/or I need to copy some headers into a new message while dropping some according to some logic. Is there anything standing in the way of these changes?