Added assignment to MString from ZString operator=.

testing/mstring_test.cpp

@@ -57,7 +57,15 @@ int main(int argc, char **argv) {
    std::cout << ">" << test10 << "<" << std::endl;
    test10 = "";
    std::cout << ">" << test10 << "<" << std::endl;
- 
+
+   // Assign from ZString tests.
+
+   coreutils::ZString test20("This is a zstring");
+   coreutils::MString test21;
+   test21 = test20;
+   std::cout << "mstring = zstring: [" << test21 <<  "]" << std::endl;
+   
+   
    return 0;
 }