Fix underscore inflector handling of adjacent acronyms

I suspect that positive lookbehind would have been used in the
original implementation had it been available in supported Ruby
versions at the time. Now that Rails requires Ruby 1.9.2 or above,
this is no longer an issue.

This fixes #14146 for acronyms such as APIRESTful. This technique also
addresses namespaced acronyms that are not entirely uppercased. This
was broken when the commit was originally written but has since been
fixed in ccbb481. The latter does not deal with adjacent acronyms so
this commit wins.
Merge pull request #14146 from chewi/fix-underscore-acronyms-regex

Fix underscore inflector handling of namespaced and adjacent acronyms