Exec detach

[bamboocleaver]

What did you implement:

A fix for Exec::start() that expected the connection to be upgraded to TCP stream unconditionally.

How did you verify your change:

I use it in my project and it passes existing tests and an additional test specifically made to check my change (container_exec_detach).

[bamboocleaver]

Thanks for your suggestions :) I applied them and updated the changelog.

[vv9k]

LGTM, thank you :)