buffer: fix case of one buffer passed to concat

Fix Buffer.concat() so a copy is always returned regardless of the
number of buffers that were passed.

Previously if the array length was one then the same same buffer was
returned. This created a special case for the user where there was a
chance mutating the buffer returned by .concat() could mutate the buffer
passed in.

Also fixes an inconsistency when throwing if an array member was not a
Buffer instance. For example:

    Buffer.concat([42]);      // Returns 42
    Buffer.concat([42, 1]);  // Throws a TypeError

Now .concat() will always throw if an array member is not a Buffer
instance.

See: https://github.com/nodejs/io.js/issues/1891
PR-URL: https://github.com/nodejs/io.js/pull/1937


Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>