stream: Do not call endReadable on a non-empty stream

Say that a stream's current read queue has 101 bytes in it, and the
underlying resource has ended (ie, reached EOF).

If you do something like this:

    stream.read(100); // leave a byte behind
    stream.read(0); // read(0) for some reason

then the read(0) will get 0 from the howMuchToRead function.  Since the
stream was ended, this was incorrectly treating the 0 as a "there is no
more in the buffer", and emitting 'end' before that last byte was read.

Why have the read(0) in the first place?  We do this in some cases to
trigger the last few bytes of a net socket (such as a child process's
stdio pipes).  This was causing issues when piping a `git archive` job
to a file: the resulting tarball was incomplete, because it occasionally
was not getting the last chunk.