Fix regression in the HTTP 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: 29983827
Test: CtsLibcoreOkHttpTestCases and CtsLibcoreTestCases
Change-Id: I117bca1371e5311b85e0cee0d3d2528191ade182
(cherry picked from commit cd57d9eb)