Does it work with something like Postman? I'd probably look at it with something like JOBEXAM to see what line of code it is executing. Maybe there is something specific browser based that it is passing that you have to define explicitly in the HTTP request.  Also suggest to try in a later Cache or IRIS version.
If you need help debugging feel free to open a WRC case with us.

-Len

Hi Brian,

This maybe fixed by CMT1677 - Correct a fault that led to spurious event log messages (incorrectly) warning about the absence of an HTTP request method.  We would need to take a closer look to see if this is the case or not.  When you get a chance please open a WRC case or give us a call (+1 617 621 0700) and we will have someone investigate.  CMT1677 is currently not in any released version of our products.

The method returns a %Status.

I reviewed the code between Cache 2013.1 and latest and I don't see any changes to the method:

<Method name="OnHTTPHeader">
<Description><![CDATA[
Event handler for <b>PAGE</b> event: this is invoked in order to  
send HTTP headers. The default action is to invoke the WriteHTTPHeader()
of the <class>%CSP.Response</class> which generates HTTP 1.0 standard
headers. Set <var>OutputBody</var> to 0 to prevent prevent <method>OnPage</method>
from being called, leave it unchanged otherwise. Returns a <class>%Status</class> code.]]></Description>
<Abstract>1</Abstract>
<ClassMethod>1</ClassMethod>
<FormalSpec><![CDATA[&OutputBody:%Boolean]]></FormalSpec>
<ReturnType>%Status</ReturnType>
<ServerOnly>1</ServerOnly>
<Implementation><![CDATA[]]></Implementation>
</Method>
 

Is there a particular problem you're running into?  Please feel free to open a WRC issue with support and we can investigate.