field name can not by empty

[justjavac]

header-name can not by empty string

https://tools.ietf.org/html/rfc7230#section-3.2

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

Browser Compatibility:

Firefox:

let headers = new Headers();
headers.set('', 'foo');  // TypeError:  is an invalid header name.

Chrome:

let headers = new Headers();
headers.set('', 'foo');  // TypeError: Failed to execute 'set' on 'Headers': Invalid name

[keithamus]

LGTM! @mislav?

[mislav]

Looks good; thank you!