The method #initialize_copy should be private, no direct call

Fixed test/t/array.rb and test/t/string.rb in standard tests.
And mrbgems/mruby-struct/test/struct.rb as well.

mrbgems/mruby-struct/test/struct.rb

@@ -169,7 +169,7 @@ assert("Struct#initialize_copy requires struct to be the same type") do
     Struct.__send__(:remove_const,:Test)
     Struct.new("Test", :a, :b)
     assert_raise(TypeError) do
-      a.initialize_copy(Struct::Test.new("a", "b"))
+      a.__send__(:initialize_copy, Struct::Test.new("a", "b"))
     end
   ensure
     Struct.__send__(:remove_const,:Test)

test/t/array.rb

@@ -208,7 +208,7 @@ end
 
 assert('Array#initialize_copy', '15.2.12.5.16') do
   a = [1,2,3]
-  b = [].initialize_copy(a)
+  b = [].__send__(:initialize_copy, a)
 
   assert_equal([1,2,3], b)
 end

test/t/string.rb

@@ -494,7 +494,7 @@ end
 
 assert('String#initialize_copy', '15.2.10.5.24') do
   a = ''
-  a.initialize_copy('abc')
+  a.__send__(:initialize_copy, 'abc')
 
   assert_equal 'abc', a
 end