Rephrase exception messages related to non empty header values

[sunkan]

It looks like the RFC-7230 require header-value to be at least 1 charecter long. But I might be wrong.

What I can find most clients/librarys allow sending of empty header values

• curl allow you to send empty headers
• Firefox allows it for xhr/fetch with the motivation We need it to be compliant with the XHR spec: "The empty string is legal and represents the empty header value."
• From the firefox thread it seams like Chrome also support sending and reciving empty header values
• Guzzle support sending empty header-value Empty headers support guzzle/guzzle#1915

[Zegnat]

Ha, that RFC feels a little misleading. It says:

Each header field consists of a case-insensitive field name followed by a colon (":"), optional leading whitespace, the field value, and optional trailing whitespace.

Which sure does sound like a “field value” is mandatory. But then they define field value as:

     field-value    = *( field-content / obs-fold )

Note the * there. A field-value contains 0 to infinite (RFC 5234, §3.6.) other things. Zero. An empty field-value is a valid field-value.

So yes, we should definitely allow empty headers when following the RFC that PSR-7 it written against. Nice work! 👍

[Nyholm]

Excellent. Thank you both!