Add test to clarify r36421. [Bug #5915]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@36535 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_array.rb

@@ -911,16 +911,19 @@ class TestArray < Test::Unit::TestCase
     $, = ""
     a = @cls[1, 2]
     assert_equal("12", a.join)
+    assert_equal("12", a.join(nil))
     assert_equal("1,2", a.join(','))
 
     $, = ""
     a = @cls[1, 2, 3]
     assert_equal("123", a.join)
+    assert_equal("123", a.join(nil))
     assert_equal("1,2,3", a.join(','))
 
     $, = ":"
     a = @cls[1, 2, 3]
     assert_equal("1:2:3", a.join)
+    assert_equal("1:2:3", a.join(nil))
     assert_equal("1,2,3", a.join(','))
 
     $, = ""