PartialFetchHandler produces invalid HTTP responses

Any URL that has PartialFetchHandler involved produces invalid HTTP responses.

This was reported against the Nexus APT community plugin here: https://github.com/sonatype-nexus-community/nexus-repository-apt/issues/47

I looked into this, and it looks like PartialFetchHandler sends an invalid Content-Range header here: https://github.com/sonatype/nexus-public/blob/82d58ec7e1464cf25efca423bbe5f8cbd6eb4f55/components/nexus-repository/src/main/java/org/sonatype/nexus/repository/http/PartialFetchHandler.java#L119

According to the RFC (https://tools.ietf.org/html/rfc7233#section-4.2) the Content-Range header needs a unit specifier of "bytes" in it.

Also, it seems like the base code doesn't correctly handle the If-Range header. I have a PR that fixes both issues if you want to merge to the base code: mpoindexter/nexus-public#1