NEXUS-6763: Per-Request checksum policy override chatty

As the condition (or the override, as you look at it) was at
wrong place. The code did not expect to start with policy
IGNORE which became possible with the per-request overrides

Ported from
1b28867af208b48137a3cee081c5f9bd3bcfda62
Merge pull request #697 from sonatype/NEXUS-6763-prefixes-checksum

NEXUS-6763: Per-Request checksum policy override chatty