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
Merge pull request #695 from sonatype/nexus-2.10.x-NEXUS-6763-prefixes-checksum

[2.10] NEXUS-6763: Per-Request checksum policy override chatty