Update httpbin endpoint on tests

[soulseekah]

The default REQUESTS_TEST_HOST endpoint (requests-php-tests.herokuapp.com) points to a very outdated version of httpbin forcing a couple of tests to be run directly against https://httpbin.org instead.

The tests are:

7b099c5 - /status/307 redirect issue fixed in later versions (see #7)
3ebc94a - /post does not show form data and file data at the same time in older versions

Any chance that app can be updated so tests can use the httpbin() function in the newer cases?

[rmccue]

Just noting that the Heroku app is not the same as httpbin.org, but rather a custom one which runs https://github.com/RequestsPHP/test-server - this exists because httpbin.org doesn't support all the tests we need.

If httpbin.org does now support those, I'd be very up for removing that custom server, as it's not great.

[carlalexander]

Just wanted to leave a note that the tests I'm writing right now for #454 only seem to work against the test server when it's run locally. Heroku seems to strip the header I'm testing against. I've also tried httpbin.org. So I don't think we can remove the test server at this time.

[ozh]

After switching to Github Actions, it should be a breeze to simply docker run -p 80:80 kennethreitz/httpbin and run tests locally