Update httpotion to 3.1.3

[iilyak]

Overview

There were couple of hacks in test/elixir/lib/couch.ex
We've got changes needed to remove them into httpotion 3.1.3.
The changes were introduced in:

• Pass headers to process response* functions valpackett/httpotion#118
• Add process_request_headers/3 to pass options valpackett/httpotion#130

This PR updates httpotion to 3.1.3 and removes the hacks from couch.ex

Testing recommendations

1. Make sure tests pass make elixir
2. Make sure we set timeouts in calls to ibrowse
   2.1. Start iex iex -S mix
   2.2. Make sure calls to ibrowse have .....{inactivity_timeout,55000}], 60000)

# Start the default trace message receiver
:dbg.tracer()
# Setup call (c) tracing on current process
:dbg.p(:erlang.pid_to_list(self()), :c)
# Setup an exception return trace (x) on ibrowse:send_req
:dbg.tp(:ibrowse, :send_req, :x)
Couch.request!(:post, "http://google.com", [])

Related Issues or Pull Requests

PRs in httpotion:

• Pass headers to process response* functions valpackett/httpotion#118
• Add process_request_headers/3 to pass options valpackett/httpotion#130

Checklist

• Code is written and works correctly
• Changes are covered by tests
• Any new configurable parameters are documented in rel/overlay/etc/default.ini
• A PR for documentation changes has been made in https://github.com/apache/couchdb-documentation

[nickva]

Are we still setting the proper httpotion/ibrowse timeouts, where does that happen:

    timeout =
      Keyword.get(
        options,
        :timeout,
        Application.get_env(:httpotion, :default_timeout, @request_timeout)
      )

    ib_options =
      Keyword.merge(
        Application.get_env(:httpotion, :default_ibrowse, []),
        Keyword.get(options, :ibrowse, [{:inactivity_timeout, @inactivity_timeout}])
      )

I had also noticed new unused tags warnings:

warning: module attribute @inactivity_timeout was set but never used
  test/elixir/lib/couch.ex:59
warning: module attribute @request_timeout was set but never used
  test/elixir/lib/couch.ex:58

[iilyak]

@nickva I updated the PR and testing instructions