Mint passive mode

[teamon]

tl;dr

Switch Tesla.Adapter.Mint to use passive mode, as suggesting in #357 (comment)

(Potentially) related issues:

• Mint adapter and :unknown messages #357
• Mint reduce_responses doesn't handle all possible responses #450
• Mint adapter cannot upload more than 65535 bytes on HTTP/2 #394
• Same request fails with some adapters and succeeds with others #385
• Mint adapter does not support HTTP2 push #370

[fbergero]

@teamon I'd like to tackle this one

[fbergero]

@teamon We basically want to change the default active mode to passive here https://github.com/elixir-tesla/tesla/blob/master/lib/tesla/adapter/mint.ex#L51 right?

Doing that only breaks some tests about how passive mode returns timeout. I'll fix those but can't think of any other test to add for this feature.

[teamon]

Basically yes :)

In case of tests it would be great to have failing test cases for all related issues and then check if changing to passive mode fixes them.