Stringify anything passed to Frame#split

Lots of things are passed to Frame#split, and errors are hard to track
down without know the context anyway. So hopefully this helps rather
than hinders.

lib/remedy/frame.rb

@@ -193,7 +193,7 @@ def sizeof content
     end
 
     def split line
-      line.split(/\r\n|\n\r|\n|\r/)
+      line.to_s.split(/\r\n|\n\r|\n|\r/)
     end
   end
 end