In some cases it is importatn to be able to keep in memory some
additional HTTP parameters. I've foudn at least one example of server
where the base URI contains parameters that MUST be kept in order to
this server to work properly...

I'm now able to do that. When building my source, I keep the WMS
paramters I need, drop the one I don't need, and keep all the other
parmaeters in another, dedicated, map. It seems to be working...
Change the way I initialize the WMS URI.
The constructor is safer than URI.create : it performs
additional validation.