close request stream after writing the body to it

Without the fix, the included test hangs when the server attempts to read the request body.

However, this is still broken:

```
$ go test
--- FAIL: TestConnPostShortBody (0.00 seconds)
	readrequest_test.go:352: Response: Header = map[Content-Type:[text/css]] want map[Content-Length:[5] Content-Type:[text/css]]
	readrequest_test.go:352: Response: ContentLength = -1 want 5
FAIL
exit status 1
FAIL	github.com/kr/spdy	0.041s
```

Missing a `Content-Length` header in the response, and the `ContentLength` value is set to -1.