IgnoreUnresolvedVariable Property issue in ServiceCallout

[ahemanna]

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<ServiceCallout async="false" continueOnError="false" enabled="true" name="SC">
    <DisplayName>SC</DisplayName>
    <Properties/>
    <Request clearPayload="false" variable="myRequest">
        <IgnoreUnresolvedVariables>true</IgnoreUnresolvedVariables>
                <Copy source="request">
             <Headers>
            <Header name="Content-Type">{request.header.Content-Type}</Header>
            </Headers>
            <QueryParams/>
        </Copy>
    </Request>
    <Response>calloutResponse</Response>
    <HTTPTargetConnection>
        <URL>http://This is a dummy endpoint</URL>
    </HTTPTargetConnection>
</ServiceCallout>

We are trying to copy content-type header from the request .In a scenario where the header is not passed in the request we get the below error inspite of having <IgnoreUnresolvedVariables>true</IgnoreUnresolvedVariables>

Error:Unresolved variable : request.header.Content-Type

Is this a bug or an expected behavior?

@Dino-at-Google @Dino @Anil Sagar @ Google @Siddharth Barahalikar