Enable reuse of LinkBuilder

Currently the linkbuilder will do the 'wrong' thing if it is reused, because it does not copy the map of parameters on .build).

This patch copies the parameters on build() and buildRelative() - and tests those.

Note that test copyOnRelativeBuild() fails (and is therefore ignored) because of a bug in
org.apache.cxf.jaxrs.utils.HttpUtils.relativize(URI, URI)