added basic support for MockQueue#unsubscribe

[jsaedtler]

Hi Dan,

i came about an issue when using moqueue on different specs in another project where moqueue is used to mock the mq parts. It raised the DoubleSubscribeError even after calling unsubscrube on the mocked queue.

After browsing your source i found that unsubscribe is implemented rather small, so i tried to fix it. It works for me and i added an spec.

What do you think of my proposal?

Thanks Jörg

[edemkumodzi]

Hey @danielsdeleo,

I'm currently facing the same issue @jsaedtler described above. Could this be merged?