Ensemble soap client misses soapaction http header

So I have an ensemble soap client which I use to send a message. The target web service is, however, giving a soapfault stating that my message misses the "SoapAction http header".

When I copy paste the message from my Cache Soap Log and fire it from Soap UI I get a proper response from the target web service. This suggests that my intersystems installation is somehow not including or losing some http headers. 

I am running Ensemble 2016.2 SUSE on an ubuntu machine.

Here is the InterSystems message with the http headers from Soap UI:

POST https://test.salesforce.com/services/Soap/c/40.0/0DF8E0000004Ctp HTTP/1.1
Accept-Encoding: gzip,deflate
Content-Type: text/xml;charset=UTF-8
SOAPAction: ""
Content-Length: 447
Host: test.salesforce.com
Connection: Keep-Alive
User-Agent: Apache-HttpClient/4.1.1 (java 1.5)

<?xml version="1.0" encoding="UTF-8" ?>
<SOAP-ENV:Envelope xmlns:SOAP-ENV='http://schemas.xmlsoap.org/soap/envelope/' xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance' xmlns:s='http://www.w3.org/2001/XMLSchema'&gt;
  <SOAP-ENV:Body><login xmlns="urn:enterprise.soap.sforce.com"><username xsi:type="s:string">test</username><password xsi:type="s:string">test</password></login></SOAP-ENV:Body>
</SOAP-ENV:Envelope>