add symbol style to Enumerable#inject

mrblib/enum.rb

@@ -199,7 +199,12 @@ module Enumerable
   #
   # ISO 15.3.2.2.11
   def inject(*args, &block)
-    raise ArgumentError, "too many arguments" if args.size > 2
+    raise ArgumentError, "too many arguments" if args.size > 3
+    if Symbol === args[-1]
+      sym = args[-1]
+      block = ->(x,y){x.send(sym,y)}
+      args.pop
+    end
     if args.empty?
       flag = true  # no initial argument
       result = nil
@@ -218,6 +223,7 @@ module Enumerable
     }
     result
   end
+  alias reduce inject
 
   ##
   # Alias for collect