Merge pull request #1723 from akuroda/string_test_cap_down

add tests for String#capitalize!, String#downcase!, and String#upcase!

test/t/string.rb

@@ -125,6 +125,7 @@ assert('String#capitalize!', '15.2.10.5.8') do
   a.capitalize!
 
   assert_equal 'Abc', a
+  assert_equal nil, 'Abc'.capitalize!
 end
 
 assert('String#chomp', '15.2.10.5.9') do
@@ -204,6 +205,7 @@ assert('String#downcase!', '15.2.10.5.14') do
   a.downcase!
 
   assert_equal 'abc', a
+  assert_equal nil, 'abc'.downcase!
 end
 
 assert('String#each_line', '15.2.10.5.15') do
@@ -442,6 +444,7 @@ assert('String#upcase!', '15.2.10.5.43') do
   a.upcase!
 
   assert_equal 'ABC', a
+  assert_equal nil, 'ABC'.upcase!
 end
 
 # Not ISO specified