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/string_tests.py b/Lib/test/string_tests.py
index 1729999..a071f20 100644
--- a/Lib/test/string_tests.py
+++ b/Lib/test/string_tests.py
@@ -314,3 +314,9 @@
test('__contains__', 'asdf', True, 'asdf') # vereq('asdf' in 'asdf', True)
test('__contains__', 'asd', False, 'asdf') # vereq('asdf' in 'asd', False)
test('__contains__', '', False, 'asdf') # vereq('asdf' in '', False)
+
+def run_inplace_tests(constructor):
+ # Verify clearing of SF bug #592573
+ s = t = constructor('abc')
+ s += constructor('def')
+ verify(s != t, 'in-place concatenate should create a new object')