bring XWIKI-6106 into curriki

As used in CheckValid's request, we need to output headers that allow caching at the client (and proxies). A venerably old bug I found, XWIKI-6106, prevented that: setHeader added to the header.

We should port the (very easy) fix to the current curriki's xwiki.
I have done so currently, using the version name:

<platform.core.version>1.5-curriki-6-SNAPSHOT</platform.core.version>

and this is deployed on current.

Let us validate that things work there and we'll allow ourselves to add more cache headers.