net/http: Transport should de-UTF-8 hostname in Host header

From golang-nuts, a test was failing in Go 1.6 against httptest.Server because of a unit test line:

``` go
    {"http://Doppel.Gänger/foo_Bar-Foo", []byte(`T€st Cont€nt 123`), false},
```

I suspect the http client is sending the "ä" as literal UTF-8 bytes in the Host header, not as punycode or whatever.
