Fixing small files fetching

[tomusdrw]

Last chunk could remain unflushed - forcing flush when all chunks from response are read
Also:

1. Adding User-Agent out of courtesy
2. Returning an error on non-200 response.

[coveralls]

Coverage decreased (-0.04%) to 86.239% when pulling b629520 on https-fetch into 3b8247b on master.

[jacogr]

Works as expected now, small files no more an issue.

[coveralls]

Coverage decreased (-0.02%) to 86.236% when pulling 7659e64 on https-fetch into 20e1d57 on master.