Unable to pass channels across more than one task

[robarnold]

Test case is approximately as follows:

1. create a channel and pass it to a new task
2. have the new task pass the channel to another new task
3. from that newest task, send on the channel
   4.receive on the original task

Expected: successful exit of program
Actual:

• warning: '_unread == 0', at: ../src/rt/circular_buffer.cpp:28 freeing circular_buffer with 4 unread bytes
• program does not terminate

[eholk]

I've written a testcase for this issue. Rob, can you look over it and make sure it represents your scenario?

https://gist.github.com/1029972