org.jboss.resteasy.specimpl.ResponseBuilderImpl.lastModified does not format date correctly.
This is regression between 2.3.5.Final and 3.0.1.Final.
In 2.3.5.Final the code did date formatting correctly (however null date https://issues.jboss.org/browse/RESTEASY-630 was not handled):
In 3.0.1.Final it does not do formatting correctly, so header value is just a toString of Date object (however null Date https://issues.jboss.org/browse/RESTEASY-630 is fixed):
It has been caused by this commit:
Fixed code should follow the form of "expires(Date)" method in the same class to both format date correctly and handle null date.
Same bug as this was also in past, in version 1.0.2: