Simplify finding warnings for assert_warning
Simplify finding warnings for assert_warning

also fixes a weird issue in Ruby 1.8.7 where duplicate
warnings were getting reported when there weren't any

jlecour/brakeman,jlecour/brakeman,cheerfulstoic/brakeman,jeffyip/brakeman,spcoder/brakeman,bf4/brakeman,bf4/brakeman,jeffyip/brakeman,lee-leon/brakeman,jakeatwork/brakeman,cheerfulstoic/brakeman,lee-leon/brakeman,masarakki/brakeman,liqites/brakeman,jakeatwork/brakeman,codeclimate/brakeman-deprecated,dfens/brakeman,github/brakeman,spcoder/brakeman,htdc/brakeman,masarakki/brakeman,dfens/brakeman,jakeatwork/brakeman,bf4/brakeman,htdc/brakeman,spcoder/brakeman,htdc/brakeman,lee-leon/brakeman,liqites/brakeman,cheerfulstoic/brakeman,masarakki/brakeman,liqites/brakeman,jlecour/brakeman,kbrock/brakeman,kbrock/brakeman,kbrock/brakeman,github/brakeman,dfens/brakeman,codeclimate/brakeman-deprecated