Implement `Array#union` which is introduced in Ruby2.6.

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

@@ -137,6 +137,25 @@ class Array
     ary.uniq! or ary
   end
 
+  ##
+  # call-seq:
+  #    ary.union(other_ary,...)  -> new_ary
+  #
+  # Set Union---Returns a new array by joining this array with
+  # <i>other_ary</i>, removing duplicates.
+  #
+  #    ["a", "b", "c"].union(["c", "d", "a"], ["a", "c", "e"])
+  #           #=> ["a", "b", "c", "d", "e"]
+  #
+  def union(*args)
+    ary = self.dup
+    args.each_with_index do |x,i|
+      ary.concat(x)
+      ary.uniq! if i % 20 == 0
+    end
+    ary.uniq! or ary
+  end
+
   ##
   # call-seq:
   #    ary & other_ary      -> new_ary

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

@@ -75,6 +75,14 @@ assert("Array#|") do
   assert_equal [1, 2, 3, 1], a
 end
 
+assert("Array#union") do
+  a = [1, 2, 3, 1]
+  b = [1, 4]
+  c = [1, 5]
+
+  assert_equal [1, 2, 3, 4, 5], a.union(b,c)
+end
+
 assert("Array#&") do
   a = [1, 2, 3, 1]
   b = [1, 4]