Fixed bug in r_string: end of string pointer was not updated in the force member function

This could lead to undefined behaviour in the std::string conversion
operator, when using the iterator interface (begin() / end()), and the size()
member function.

include/crow/json.h

@@ -168,9 +168,10 @@ namespace crow
                     return os;
                 }
             private:
-                void force(char* s, uint32_t /*length*/)
+                void force(char* s, uint32_t length)
                 {
                     s_ = s;
+                    e_ = s_ + length;
                     owned_ = 1;
                 }
                 friend rvalue crow::json::load(const char* data, size_t size);