Update basic_string with missing C++11 and C++17 interfaces:

- Default npos arguments in append, insert, compare
- Initializer list support
- Non-const data()

test/string_test.cpp

@@ -183,6 +183,10 @@ int string_test()
          stdStringVect->push_back(auxStdString);
       }
 
+      if(auxBoostString.data() != const_cast<const BoostString&>(auxBoostString).data() &&
+         auxBoostString.data() != &auxBoostString[0])
+         return 1;
+
       if(!CheckEqualStringVector(boostStringVect, stdStringVect)){
          return 1;
       }