Is HSTS disabled for a reason? Can't set the Strict-Transport-Security header

[Kenneth]

I was playing around with this site: https://www.trustworthyinternet.org/ssl-pulse/ and noticed that my app engine site, (which gets an A, well done Google but I expect nothing less :-) wasn't listed as supporting HSTS when I thought it did.  So I investigated and found that my HSTS header which used to work doesn't work any more.  I have no idea when it stopped working, need to add it my test suite.

Is there a reason for this?  It isn't in the blacklist here: https://developers.google.com/appengine/docs/python/tools/webapp/responseclass#Disallowed_HTTP_Response_Headers

Issue entered:

http://code.google.com/p/googleappengine/issues/detail?id=7427 

thanks.