stream: unshift('') is a noop

In some cases, the http CONNECT/Upgrade API is unshifting an empty
bodyHead buffer onto the socket.

Normally, stream.unshift(chunk) does not set state.reading=false.
However, this check was not being done for the case when the chunk was
empty (either `''` or `Buffer(0)`), and as a result, it was causing the
socket to think that a read had completed, and to stop providing data.

This bug is not limited to http or web sockets, but rather would affect
any parser that unshifts data back onto the source stream without being
very careful to never unshift an empty chunk.  Since the intent of
unshift is to *not* change the state.reading property, this is a bug.

Fixes #5557
Fixes LearnBoost/socket.io#1242