Adding streaming http support.

[hzeng-otterai]

This pack adds the streaming support for http.
The responses lib doesn't support mock of streaming http so I didn't have a good way to unit test it. Any suggestions?

[menshikh-iv]

Hi @handsomezebra, I think you can mock it manually with standart python mock.

Also, it's a good idea to add some integration test too (just up simple HTTP server and read file from it)

CC: @mpenkov

[menshikh-iv]

Looks good, thanks @handsomezebra, please work on testing now.

[mpenkov]

In general, the code looks very good.

I think we need more tests, especially unit tests for the new SeekableBufferedInputBase class. Seek logic can be tricky, and having tests will insure us against logic bugs in that functionality.

[menshikh-iv]

ping @handsomezebra, are you planning to finish PR?
CC @mpenkov

[mpenkov]

Still need to add tests.

[mpenkov]

@handsomezebra I cleaned up your commit for you, added some tests, and merged this in. Congrats on your first PR to smart_open 🚀