Update downloaders.py

Fix Issues #283 .srt and .txt files problem
Merge pull request #287 from zlbruce/patch-1

Fix #283, where .srt and .txt files were being written in gzipped format.

The problem here is that we were using the raw response of
`urllib3.response.HTTPResponse` via the `requests` library,
which preserved the transfer encoding of the text files.

Thanks @zlbruce for the fix.