better callstack reporting in deprecation messages

now the reported line is the first line in the stack
that's outside Rails, which is the one that actually
caused the problem in the first place

[#5231 state:resolved]

Signed-off-by: José Valim <jose.valim@gmail.com>

activesupport/lib/active_support/deprecation/reporting.rb

@@ -46,10 +46,14 @@ module ActiveSupport
         end
 
         def extract_callstack(callstack)
-          if md = callstack.first.match(/^(.+?):(\d+)(?::in `(.*?)')?/)
-            md.captures
-          else
-            callstack.first
+          rails_gem_root = File.expand_path("../../../../..", __FILE__) + "/"
+          offending_line = callstack.find { |line| !line.start_with?(rails_gem_root) } || callstack.first
+          if offending_line
+            if md = offending_line.match(/^(.+?):(\d+)(?::in `(.*?)')?/)
+              md.captures
+            else
+              offending_line
+            end
           end
         end
     end