Added ZString constructor for assignment on definition.

testing/mstring_test.cpp

@@ -27,6 +27,8 @@ int main(int argc, char **argv) {
 
    coreutils::ZString test7("zstring data");
    coreutils::MString test4;
+   coreutils::MString test5 = test7;
+   std::cout << "coreutils::MString test5 = test7: [" << test5 << "]" << std::endl;
    test4 << "this is a test." << test7;
    std::cout << "char* operator<<: [" << test4 << "]." << std::endl;