Regression test for HTTP proxy request line

Adds a test that demonstrates a regression from M in
the HTTP request line when a proxied connection
is made. It only affects URLs with empty paths (i.e. ones
without even a '/' after the host / authority). For
example, on M the request line for a request to
new URL(http://myhost).openConnection() would be:

GET http://myhost HTTP/1.1

but on N, without this change, it would be:

GET http://myhost/ HTTP/1.1

This change reverts to the M behavior.

Bug: 30107354
Bug: 29983827
Change-Id: Iea34afe3beb67022f746aa1b04262e43d841aacd
(cherry picked from commit d1288ce6)