http: Consistent 'finish' event semantics

In other Writable streams, the 'finish' event means that all of the data
was written, and flushed to the underlying system.

The 'prefinish' event means that end() was called, and all of the data
was processed, but not necessarily completely flushed.

This change brings the http OutgoingMessage classes more in sync with
the other Writable classes throughout Node.

Unfortunately, this change highlights an issue with http
IncomingMessages, where the _dump() method will not actually pull the
data off the wire.  This is a minor issue that is typically only
relevant in test cases, and will be addressed in the next commit.