Request Headers Validation issue in >= 2.1.1

[Ibrahim AshShohail]

PHP version: 8.1.4

Description
It seems the

How to reproduce
Building any request seems to trigger this issue.

Possible Solution
https://github.com/guzzle/psr7/blob/master/src/MessageTrait.php#L262
I'm assuming the inverse is the intention?

[M. Vugteveen]

same as #489

[Graham Campbell]

Closing since this is a duplicate.