Allow "_" in URIs.

Note that this is contrary to RFC-2396, which only allows alphanumeric
characters and hyphens. We need this change because internet hosts out
there clearly don't care enough about standards.

bug: 18023709

(cherry picked from commit 435edc3f)

Change-Id: I5a2ccad41f302b36b0703ae381f0a9954266ff26