Stream: improved async callback support for s.send().

Stream: improved async callback support for s.send().

Previously, the "from_upstream" flag (introduced in b33aae5e8dc6) was
ignored for s.send() calls invoked directly from a body filter.
This makes the s.send() behaviour context dependent.

The fix is to always take "from_upstream" flag into account when it is
provided.

This fixed #552 issue on Github.