Merge pull request #4720 from shuujii/fix-Enumerable-filter_map-without-block

Fix `Enumerable#filter_map` without block; ref d380c7d2

mrbgems/mruby-enum-ext/mrblib/enum.rb

@@ -828,7 +828,7 @@ module Enumerable
   end
 
   def filter_map(&blk)
-    return to_enum(:find_index, val) unless blk
+    return to_enum(:filter_map) unless blk
 
     ary = []
     self.each do |x|