NEXUS-5790: Reverted location of constants.

As restlet1x depends on nexus-web-utils, makes sense to have them
in latter, instead to "keep them in sync" with former.

Also, content-disposition here constant turned out to be unused,
and NexusContentServlet now always set it, as according to
RFC it's on client to use it (Maven will neglect it, while
clients like wget or curl have switches to obey or not to
this header).

This header matters most in "interactive" UAs, like
browsers and downloaders are anyway.