Implement http-client backend for stripe requests

[creichert]

fixes #90

This is some initial work I've done to implement an http-client backend for stripe requests. I wanted to get some feedback before I commit too much time to "finalizing" it for merge.

Notes:

• I used http-client instead of http-conduit. I don't use http-conduit much myself and don't really know yet what a stripe-http-conduit library may look like.
• I add stripeConn and withConnection for API compatibility so that existing clients using stripe-http-streams could plug this in without any necessary changes. However, http-client's terminology regarding "connections" is different than http-streams. I'm considering adding DEPRECATED pragmas to these specific functions to get people to use the stripeManager and withManager function to avoid confusion. I don't have strong feelings on this as I primary use stripeManager.
• The main function stripe uses an http-client "global" manager (IORef Manager) defined from http-client-tls. I feel like this is good for perf so managers aren't created often.
• I need to add a little more documentation on why using newManager (as opposed to http-client's deprecated withManager is safe with resources).
• I may make a few coding style changes to the HttpClient module to be more consistent w/ stripe-http-streams package.

Follow-Up:

• As I mentioned in #90, I want to add a flag to the stripe-haskell package for this backend. Not sure what the best way to go about the import is (CPP or creating a new Web.Stripe.Client module in stripe-http-streams & stripe-http-client`

Initial test results

Failures:

  tests/Web/Stripe/Test/Event.hs:15: 
  1) Event tests Succesfully retrieves events
       predicate failed on: Left (StripeError {errorType = ParseFailure, errorMsg = "key \"evidence_due_by\" not present", errorCode = Nothing, errorParam = Nothing, errorHTTP = Nothing})

Randomized with seed 1531494023

Finished in 193.2520 seconds
89 examples, 1 failure

[creichert]

It looks like the stripe-http-streams lib fails for the same reasons:

Failures:

  tests/Web/Stripe/Test/Event.hs:15: 
  1) Event tests Succesfully retrieves events
       predicate failed on: Left (StripeError {errorType = ParseFailure, errorMsg = "key \"evidence_due_by\" not present", errorCode = Nothing, errorParam = Nothing, errorHTTP = Nothing})

Randomized with seed 567401952

Finished in 211.6145 seconds
89 examples, 1 failure

stripe-http-streams-2.3.0: Test suite tests failed
Completed 9 action(s).
Test suite failure for package stripe-http-streams-2.3.0
    tests:  exited with: ExitFailure 1
Logs printed to console

[creichert]

@dmjio @nmattia Can y'all review the code here? I can remove the Manager or Connection functions if you want. Other than that, it's ready to merge. From there, I'll create a new PR to stripe-haskell that creates a flag for HTTP client.

  - location:
      git: https://github.com/creichert/stripe
      commit: 56f4067fbebe8298b94519327367213a05cddd5a
    extra-dep: true

...

import Web.Stripe.Client.HttpClient

[dmjio]

Bravo @creichert. Huge thanks 🍺

[nmattia]

thanks @creichert !