Low level part of fixing #403.

Make the LATEST STABLE package version the one returned by default when version == null.
Make the LATEST version the one returned if no STABLE version exists and allowPrerelease == true.
UI fixes to reflect that we now take you to the latest stable package version by default.
MINOR - test naming nit
Merge pull request #1626 from NuGet/tilovell/403-LatestStableShouldBeDefaultPackageVersionSeen

Tilovell/403 latest stable should be default package version seen