Update Password with has_mixed_case placeholder

Password.cpp

@@ -18,3 +18,12 @@ int Password::count_leading_characters(string phrase){
   }
   return repetition;
 }
+
+
+/*
+  receives a string and returns whether it has both at least one upper-case
+  letter and at least one lower-case letter
+*/
+bool Password::has_mixed_case(string pass){
+  return false;
+}