Fix folly Uri::host() return value for IPv6 address

Fix folly Uri::host() return value for IPv6 address

Summary: folly Uri::host() returns leading and trailing brackets for IPv6 host, for example, for URI "http:://[::1]:8080/index.html", host() method returns "[::1]". But square brackets are not part of host in fact, this diff is going to remove brackets for IPv6 host.

Test Plan: fbconfig -r folly/test && fbmake runtests_dbg

Reviewed By: chip@fb.com

FB internal diff: D1276513