Added test case that causes the $$& operator to deadlock

I discovered a deadlock, which occurred in the use of the
$$& operator. Here is the code snippet to reproduce the
deadlock:

C.sourceList [1..100] $$& C.fold (+) 0
*** Exception: thread blocked indefinitely in an STM transaction
fix for the deadlock with $$& operator

The problem was that the sender of a TBMChan did not sent a sentinel value to
the receiver to signalize the end of the transmission. This caused the receiver
of the channel to wait for incomming values forever.
Merge pull request #11 from svenkeidel/bug-async-operator-deadlock

Bug async operator deadlock