Upload file to thread instead of channel when in thread buffer
Upload file to thread instead of channel when in thread buffer

Currently files are always uploaded to the main channel, even when
`/slack upload` is run in a thread buffer. This happens because the
`thread_ts` field isn't included in the upload request.

Fixes #672

wee-slack/wee-slack,trygveaa/wee-slack,rawdigits/wee-slack