Add missing \r to the response headers

[Davoodeh]

https://www.rfc-editor.org/rfc/rfc7230#section-3

All HTTP/1.1 messages consist of a start-line followed by a sequence
of octets in a format similar to the Internet Message Format
[RFC5322]: zero or more header fields (collectively referred to as
the "headers" or the "header section"), an empty line indicating the
end of the header section, and an optional message body.

    HTTP-message   = start-line
                     *( header-field CRLF )
                     CRLF
                     [ message-body ]

This causes problems with some clients like docs.rs/surf