HTTP: fix missing validation of the r.return() redirect target

HTTP: fix missing validation of the r.return() redirect target

Previously, for a redirect status ngx_http_send_response() stores the
text passed to r.return() as the Location header value, but the text was
only checked when it was used as a response body, where control
characters are valid.

The text is now validated as a header value for the statuses that turn
it into Location, and is left as is otherwise.