Fix ASGITransport path escaping

[tomchristie]

In the ASGI scope, the path key should be URL escaped...

path (Unicode string) – HTTP request target excluding any query string, with percent-encoded sequences and UTF-8 byte sequences decoded into characters.

Also, since 0.15.0 we're making sure to properly URL escape path components, so we're more likely to be bumping into this bug. Eg. in the test case included here we use...

url = httpx.URL("http://www.example.org/").copy_with(path="/user@example.org")

Which from 0.15.0 onwards will result in "http://www.example.org/user%40example.org", which will trigger the ASGI bug.