Fix keys of consumer handler functions

The documentation says to use function handler keys ending in "-fn", but this code currently only recognizes the old form. This commit ensures that all keys that are used within `langohr.consumers/subscribe` can be used as a parameter.
Fix broken commit: we produce a seq of keywords, not a seq of seqs
add test to verify that  takes both versions for function handler fns
Merge pull request #22 from smee/patch-1

Fix keys of consumer handler functions