stream_wrap: error if stream has StringDecoder

If `.setEncoding` was called on input stream - all emitted `data` will
be `String`s instances, not `Buffer`s. This is unacceptable for
`StreamWrap`, and should not lead to the crash.

Fix: https://github.com/nodejs/node/issues/3970
PR-URL: https://github.com/nodejs/node/pull/4031


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>