Remove check for empty header string value

[sunkan]

I was using Buzz when I got an exception about empty header value.
When I tried to use Zend Diactoros instead of this library it worked as expected.

If you'r interested I can put together a better fix that dose the validating more like it's done in Zend Diactoros

[Nyholm]

You are correct. Thank you for this fix.

[Zegnat]

Should the Exception messages be updated after this, if this now allows empty header values? (Also interesting how this could be done without tripping up any tests, clearly something not being tested.)

[Nyholm]

Exception messages should not be updated. The PR enforce that empty headers should NOT be added.

About tests:
Yes, I'm also curious. This PR adds checks for $request->withAddedHeader('name', ['foobar', '']);
I know we have tests for $request->withAddedHeader('name', '');

[Nyholm]

Oh. I misread this PR. Martin, you are correct. We should update exception message.

[Nyholm]

Sunkan, thank you for the chat yesterday and sorry for prematurely merging this PR.

Could you improve the code by updating the exception messages and reference the RFC that specifies the header value? (Or where it says the value can be empty)

[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