Make sure to drain the websocket TextMessage

[slaskis]

Fixes the failing test I created in convox/convox#426 and should fix convox/issues#9

[heronrs]

@slaskis thanks for the PR!
So this should fix piping output through the convox exec command, right?
Is there any simple manual test I can do on my side to validate that?
Just piping a big text file should be enough?

[slaskis]

Yep that should be enough. Its a bit racy as it depends on how much was able to pass through the TextMessage io.Reader .Read call but ive seen it fail sort of reliable around 16kb and up.

And I believe it is the same code used for process input output as well :)