The parser was assuming it would find <body> or </head>.
If the entire response is just <meta> tags, it finds EOF and
treats that as an error. It's not.
This is the same change as in https://golang.org/cl/68520044.
Fixes#9556.
Change-Id: If51ed36e7364c15788311039caf8323eb5fe9a6c
Reviewed-on: https://go-review.googlesource.com/2650
Reviewed-by: Minux Ma <minux@golang.org>