NEXUS-5863: Attribute recursion fix

Attribute recursion caused by OS native path separator.
Nexus, while (sadly) does not enforce/validate this, is strict regarding
paths and used path separators.

org.sonatype.nexus.proxy.item.RepositoryItemUid.PATH_SEPARATOR

Is the only separator it "recognizes" in any path occurrence.

This change fixed a "OS separator leak" occurring when Nexus
runs on Win OSes. In general, these constructs (passing in
java.io.File path directly to Nx path) should be avoided.

Issue:
https://issues.sonatype.org/browse/NEXUS-5863