Allow empty header values

[jimmy-park]

The current implementation rejects the request and responds with 400 Bad Request if the header value is empty.
However, RFC 7230 states that empty header values are allowed.

header-field   = field-name ":" OWS field-value OWS

field-name     = token
field-value    = *( field-content / obs-fold )
field-content  = field-vchar [ 1*( SP / HTAB ) field-vchar ]
field-vchar    = VCHAR / obs-text

obs-fold       = CRLF 1*( SP / HTAB )
               ; obsolete line folding
               ; see [Section 3.2.4](https://datatracker.ietf.org/doc/html/rfc7230#section-3.2.4)

[yhirose]

@jimmy-park thanks for the pull request!