NEXUS-6249: Wrong checksums cached for Maven artifact

When they get updated. While this is not a problem
in 90% of cases (as releases nor unique snapshots never
get updated), the non-unique snapshots and maven metadata
files, once cached, their initial SHA1/MD5 remote
hashes were NEVER updated.
NEXUS-6249: Added UT verifying change

The UT simply forces Nexus to re-fetch the same file
(content or layout does not matter, it only matters it
happens in a Maven repository) and asserts that as
many times we change "version" of the content, the
retrieved digest will change too.

Before fixing this issue, Nexus would stick to the
remote hash retrieved the very first time.
Merge pull request #341 from sonatype/NEXUS-6249-m2-proxy-checksum

NEXUS-6249: Wrong checksums cached for Maven artifact