Fix: map request headers to lower case to avoid key name duplications

[VisargD]

forward_headers setting is used to explicitly pass a header from the original request to the provider. But the original request headers will always be with lower case keys when received in gateway. Due to this, when users add the same header but with different casing, it creates a duplicate key of that header in the final request object.

For example: If the request has an authorization header but the forward_headers contains the same with capital A (Authorization), the final headers object has 2 authorization key with different casing.

Solution: Streamline this by converting the final forward headers key to lower case so that there are no differences while mapping.