test is_https_redirect via public api

[T-256]

Summary

chunk of #2851

[tomchristie]

Hrm. Is there a way around that allows us to test this against public API?

[tomchristie]

Might be that we're okay with leaning on implementation details a little bit, certainly not a terrible trade-off to make. (I think we already have some instances of this in our test cases right?)

[T-256]

Also, we already use private-accessors in another test:

httpx/tests/client/test_proxies.py

Lines 333 to 338 in 4f6edf3

	def test_proxy_with_mounts():
	proxy_transport = httpx.HTTPTransport(proxy="http://127.0.0.1")
	client = httpx.Client(mounts={"http://": proxy_transport})
	transport = client._transport_for_url(httpx.URL("http://example.com"))
	assert transport == proxy_transport

[tomchristie]

Okay thanks. We're still using private API here, but I do think it's a little neater anyways since it nearly gets us to consistent imports all the way through the tests.