Separate download func to smaller units

[nappex]

We'd like to divide Onigumo.download to smaller parts with philosophy one func make one task.
This approach is more suitable for deep testing also. We'd like to use tmp_dir in test for testing files, for this intention we need to have the way how to give filepath as argument. When we reach it, we will be able to give tmp_dir as argument to make integration tests.
Onigumo.download make three different tasks at this moment.

Three tasks are:

1. make request
2. check request and match body of request to variable
3. write body content to file

[Glutexo]

Cool!

[Glutexo]

Partially addressed by #29.

[Glutexo]

Now with #29 merged, we can think about whether and how to split the download function to smaller, more atomic, function. I am a bit afraid of having too small chunks, but that’s probably the real functional way. Until now, every split has proven itself useful and good for testing.

[Glutexo]

Streams would help this a lot. See #70.

[Glutexo]

Partially addressed by #72. I am not sure yet how to split out the body extraction from the response object.

[Glutexo]

#72 eventually resolved this completely.