Fix request headers in tests

[jlomas-stripe]

My tests were bad and I should feel bad.

I was sending api_version in the header, instead of stripe_version, so it ... wasn't actually setting them.

Couldn't understand why the versions needed updating in #371 but after investigating I realized it's because I wasn't actually specifying them, and latest no longer mapped to the value I was using.

r? @brandur-stripe

[brandur-stripe]

Haha, I guess this is one the downfall of unit tests — you're testing what you already expect to occur :) Happens to all of us though.

How can you just change this without updating any mainline code though? Has the build been broken this whole time, or are the tests not actually checking anything?

[jlomas-stripe]

It broke when Stripe released 2017-08-15, so I think it's simply a result of nothing having triggered a build since before 2017-08-15 was released.

That said, the tests were 'magically working' because latest happened to equal the hard-coded version I'd thought I was passing through (but was not); once latest meant something different, it broke, so now I'm actually passing through the right parameter.

[brandur-stripe]

Nice! Okay, I think I misunderstood what we were changing initially, but totally makes sense now. Thanks!