Allow changing gender of single gender Pokemon

Before that change, Gender Stone and Gender Ball didn't actually
do anything with single-gender Pokemon such as Chansey.

Data/Scripts/014_Pokemon/001_Pokemon.rb

@@ -448,12 +448,12 @@ class Pokemon
 
   # Makes this Pokémon male.
   def makeMale
-    self.gender = 0;
+    @gender = 0
   end
 
   # Makes this Pokémon female.
   def makeFemale
-    self.gender = 1;
+    @gender = 1
   end
 
   # @return [Boolean] whether this Pokémon is male