src: don't call DecodeWrite() on Buffers

Don't call DecodeWrite() with a Buffer as its argument because it in
turn calls StringBytes::Write() and that method expects a Local<String>.

"Why then does that function take a Local<Value>?" I hear you ask.
Good question but I don't have the answer.  I added a CHECK for good
measure and what do you know, all of a sudden a large number of crypto
tests started failing.

Calling DecodeWrite(BINARY) on a buffer is nonsensical anyway: if you
want the contents of the buffer, just copy out the data, there is no
need to decode it - and that's exactly what this commit does.

Fixes a great many of the following run-time error in debug builds:

    FATAL ERROR: v8::String::Cast() Could not convert to string