HEAD command not setting content-type in response under certain circumstances

my UI developers have a jnlp file that downloads jars to run on the client. in Jetty 6.1.9 this worked for java 1.5 clients, but in 6.1.18, it does not work as expected. with java 1.5 clients, it complains that the Content-Type is null, and, it is:

This is what the client sends to the server:

mattrose@coltrane:~$ telnet 192.168.70.156 80
Trying 192.168.70.156...
Connected to 192.168.70.156.
Escape character is '^]'.
HEAD /webstart/N-central/jrcc-1.5.jar?version-id=7.0.0.888 HTTP/1.1
pragma: no-cache
accept-encoding: pack200-gzip,gzip
Content-Type: application/x-java-archive; charset=utf-8
User-Agent: JNLP/1.5 javaws/1.5.0_20 (b02) J2SE/1.5.0_20
UA-Java-Version: 1.5.0_20
Host: 192.168.101.108
Accept: text/html, image/gif, image/jpeg, ; q=.2, */; q=.2
Connection: keep-alive

and this is the response from the server:

HTTP/1.1 200 OK
Content-Type: text/html; charset=iso-8859-1
Content-Length: 1403
x-java-jnlp-version-id: 7.0.0.888
Server: Jetty(6.1.9)

But if I send the same thing to a 6.1.18 server, I get this in response:

HTTP/1.1 200 OK
x-java-jnlp-version-id: 7.0.0.892
Content-Length: 0
Server: Jetty(6.1.18)

I tried to figure out why the difference exists, so that I could patch it myself, but I couldn't figure out why the Content-Type: header wasn't getting set in this one circumstance.