net/url: use bytes.Buffer in (*URL).String

BenchmarkString before:

        11990 ns/op            1621 B/op         73 allocs/op

Using bytes.Buffer:

        8774 ns/op            1994 B/op         40 allocs/op

I also tried making a version of escape() that writes directly to the
bytes.Buffer, but it only saved 1 alloc/op and increased CPU time by
about 10%. Didn't seem worth the extra code path.

R=bradfitz
CC=golang-dev
https://codereview.appspot.com/7182050