Return the full string when requested from a flatbuffers::String, even if it contains a NULL byte.

include/flatbuffers/flatbuffers.h

@@ -379,7 +379,7 @@ template<typename T> static inline size_t VectorLength(const Vector<T> *v) {
 
 struct String : public Vector<char> {
   const char *c_str() const { return reinterpret_cast<const char *>(Data()); }
-  std::string str() const { return c_str(); }
+  std::string str() const { return std::string(c_str(), Length()); }
 
   bool operator <(const String &o) const {
     return strcmp(c_str(), o.c_str()) < 0;