array.rb: Array#uniq to return always Array.

Even called for subclass of `Array`, according to new Ruby3.0 behavior.
Other methods of `Array` behave as Ruby3.0 from the first hand.

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

@@ -51,7 +51,7 @@ class Array
   #    b.uniq { |s| s.first } # => [["student", "sam"], ["teacher", "matz"]]
   #
   def uniq(&block)
-    ary = self.dup
+    ary = self[0..-1]
     ary.uniq!(&block)
     ary
   end