npm contentUri should always end in slash for publish

In order for npm cli version 2.1.7 or higher to be happy, registry url passed to it must end in slash on *publish*.

I'm not sure if this is an npm cli regression or not, so I filed: https://github.com/npm/npm/issues/6982

This pull has one way to make our client act like an npm cli during publish.

Alternately, we can just append a slash in the [PublishIT](https://github.com/sonatype/nexus-oss/blob/nexus-2.11.x-npm-contenturi-slash/testsuite/modern-testsuite/src/test/java/org/sonatype/nexus/testsuite/npm/publish/PublishIT.java#L71-71).

Ending in slash works for all npm cli versions.

We should probably wait to see what the NPM folks say about this.