Sending HTTP headers with an `xf:submission`

form.xml

<xh:html xmlns:xh="http://www.w3.org/1999/xhtml"
      xmlns:xf="http://www.w3.org/2002/xforms"
      xmlns:xxf="http://orbeon.org/oxf/xml/xforms"
      xmlns:ev="http://www.w3.org/2001/xml-events"
      xmlns:xs="http://www.w3.org/2001/XMLSchema"
      xmlns:fr="http://orbeon.org/oxf/xml/form-runner">
    <xh:head>
        <xf:model>
            <xf:instance>
                <first-name xmlns=""/>
            </xf:instance>
            <xf:submission
                id="my-submission"
                ref="instance()"
                resource="http://httpbin.org/post"
                method="post"
                replace="all">
                <xf:header>
                    <xf:name>My-Name</xf:name>
                    <xf:value>My-Value</xf:value>
                </xf:header>
            </xf:submission>
        </xf:model>
    </xh:head>
    <xh:body>
        <xf:trigger>
            <xf:label>Submit</xf:label>
            <xf:send event="DOMActivate" submission="my-submission"/>
        </xf:trigger>
    </xh:body>
</xh:html>