Moved inplace add and multiply methods from UserString to MutableString.
Closes SF Bug #592573 where inplace add mutated a UserString.
Added unittests to verify the bug is cleared.
diff --git a/Lib/test/test_string.py b/Lib/test/test_string.py
index c92f5f7..a30cdbd 100644
--- a/Lib/test/test_string.py
+++ b/Lib/test/test_string.py
@@ -52,6 +52,7 @@
string_tests.run_module_tests(test)
string_tests.run_method_tests(test)
string_tests.run_contains_tests(test)
+string_tests.run_inplace_tests(str)
string.whitespace
string.lowercase