Controllers with acronyms in their names (e.g. PDFController) require the correct default helper (PDFHelper in file pdf_helper.rb).  Closes #2262.  Do not raise an exception when default helper is missing; log a debug message instead.  It's nice to delete empty helpers.

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@2938 5ecf4fe2-1ee6-0310-87b1-e25e094e27de