[2.7] NEXUS-6179: Fix redirect strategy index page detection

Clearing up what is the code expectation, how we detect is
redirect leading to an index page or not (by linking a Google
blog entry explaining it).

Improving code to be "schema switch" or "hostname switch"
resilient, by basically only assuming that remote is properly
set up and index pages will lead to URLs having paths
with trailing slash.

Also, using parsed URIs for this, to circumvent any possible
query parameter in redirect response (something old code
did not account for).

Issue
https://issues.sonatype.org/browse/NEXUS-6179

CI
https://bamboo.zion.sonatype.com/browse/NX-OSSF1