Req.Request, Req.Response: Change headers to be maps

[wojtekmach]

Closes #223

[florish]

Hi @wojtekmach, thanks for all the work put into Req!

We're in the processing of migrating from 0.3 to 0.4, and in the process I assumed that the :headers option in Req.new/1 and Req.update/2 would also have been changed to expect map data instead of lists. E.g.:

req = Req.new(url: "https://example.com", headers: %{"accept" => ["application/json"]})
Req.update(req, headers: %{"foo" => ["bar"]})

Is there a specific reason that these values are still lists in 0.4.x?

[wojtekmach]

Both are supported! (If maps are not, it’s a bug.) the idea is passing eg accept: “application/json” is much more concise than the equivalent map with, notably, list of values.

[wojtekmach]

In hindsight this is not necessarily super obvious so a PR that updates docs is welcome as always!

[florish]

Ah, thanks! Would you like me to replace the existing list examples by maps or would you prefer to have examples added using maps?

[florish]

@wojtekmach See #241 for PR, feel free to change anything you like!