Non lowercase header validation

[juvaly]

When using this configuration:
validation: { headers: { 'x-app-key': { isRequired: true } } }

A request using the required field works fine:
curl -X POST -H "Content-Type: application/json" -H "x-app-key: 123123"

However, when trying to use uppercase letters in the header name:
validation: { headers: { 'X-app-key': { isRequired: true } } }

A similar request:
curl -X POST -H "Content-Type: application/json" -H "X-app-key: 123123"

Throws:
{"code":"InvalidArgument","message":"X-app-key (MISSING): Field is required"}