Fix StandardAnalyzer to not mis-identify HOST as ACRONYM by default

Coming out of the discussion around back compatibility, it seems best to default StandardAnalyzer to properly fix LUCENE-1068, while preserving the ability to get the back-compatible behavior in the rare event that it's desired.

This just means changing the replaceInvalidAcronym = false with = true, and, adding a clear entry to CHANGES.txt that this very slight non back compatible change took place.

Spinoff from here:

http://www.gossamer-threads.com/lists/lucene/java-dev/57517#57517

I'll commit that change in a day or two.