Stop messing with people's charsets

TL;DR: Forcing the response's encoding breaks `/attach` when multiplexing is activated.

Long story:

When you `/attach` a container that has `Config.Tty` set to `false`, docker enables multiplexing of the response in order to differenciate `stdin`, `stdout` and `stderr`. Multiplexing consist in a simple enough protocol: 1 byte for the stream type, 3 null bytes, 4 bytes for the content length.

Now here's my understanding of what happens when you force the reponse's encoding to `utf8` (not sure cause I don't really know how node internals work in this regard): node treats the response data as a string and tries to convert it from whatever encoding it thinks it is to `utf8`. In my case, it converted the last byte (`164`) to an unicode triplet (`239`,`191`,`189`). Things get fun when `stream.read(8)` starts returning 10 bytes.

Anyway, this is really annoying, and for a library that claims to "NOT break any stream" quite unfortunate too :-)