Fix HeadersValidator for parameters with content

[pboos]

Headers can also have content instead of schema the same as other parameters (see OpenAPI Specification here).
This is handled correctly in other parameters but seems to have gotten missing in the HeadersValidator. For example see the ArrayValidator [link], which uses the same code line as I updated the HeaderValidator to.

Without this change it causes an exception:

TypeError: League\OpenAPIValidation\Schema\SchemaValidator::validate(): Argument #2 ($schema) must be of type cebe\openapi\spec\Schema, null given, called in ***/vendor/league/openapi-psr7-validator/src/PSR7/Validators/HeadersValidator.php on line 44

[pboos]

@scaytrase would be great if this could be merged as we are encountering this problem at @ GetYourGuide.

[oliviersorine]

I encounter the same issue on my project and found the same fix, it would be great if this PR is validated !