Add basic implementation for mandatory publishes and return handlers

[jwoodrow]

Hi 👋

I know this is very very very basic (I'm not event mocking a Bunny::BasicReturn) but I had the need to test my on_return behaviour when publishing messages with a mandatory: true option.

It works as is but:

• Would be better to emulate Bunny's BasicReturn logic (I couldn't figure out how)
• Could not find where to add some specs for this

Methods added:

• on_return(&block) : public method on Bunny::Exchange (exact same implementation as Bunny)
• handle_return(basic_return, properties, content) : private method on Bunny::Exchange

Methods updated:

• deliver for Direct, Topic, Fanout (Headers does not work as intended so I haven't changed anything)
  • If opts[:mandatory] is true then deliver will call handle_return to notify that the message could not be delivered.