net/url: prepend slash to path in String()

net/url: prepend slash to path in String()

Previously if a path was set manually without a leading /, String()
would not insert the slash when writing its output. This would lead
to situations where a URL that should be http://www.google.com/search
is output as http://www.google.comsearch

Fixes #5927.

R=golang-dev, bradfitz, rsc, 0xjnml
CC=golang-dev
https://golang.org/cl/11698045