stream: use more explicit statements

Using objectMode with stream_wrap has not worked properly
before and would end in an error.
Therefore prohibit the usage of objectMode alltogether.

This also improves the handling performance due to the
cheaper chunk check and by using explicit statements as they
produce better code from the compiler.

PR-URL: https://github.com/nodejs/node/pull/13863
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

lib/internal/errors.js

@@ -171,7 +171,7 @@ E('ERR_SOCKET_BAD_PORT', 'Port should be > 0 and < 65536');
 E('ERR_SOCKET_DGRAM_NOT_RUNNING', 'Not running');
 E('ERR_STDERR_CLOSE', 'process.stderr cannot be closed');
 E('ERR_STDOUT_CLOSE', 'process.stdout cannot be closed');
-E('ERR_STREAM_HAS_STRINGDECODER', 'Stream has StringDecoder');
+E('ERR_STREAM_WRAP', 'Stream has StringDecoder set or is in objectMode');
 E('ERR_TRANSFORM_ALREADY_TRANSFORMING',
   'Calling transform done when still transforming');
 E('ERR_TRANSFORM_WITH_LENGTH_0',