stream: only end reading on null, not undefined

The [Stream documentation for .push](http://nodejs.org/api/stream.html#stream_readable_push_chunk_encoding

)
explicitly states multiple times that null is a special cased value
that indicates the end of a stream. It is confusing and undocumented
that undefined *also* ends the stream, even though in object mode
there is a distinct and important difference.

The docs for Object-Mode also explicitly mention null as the *only*
special cased value, making no mention of undefined.

Signed-off-by: Fedor Indutny <fedor@indutny.com>