if define RAPIDJSON_HAS_STDSTRING, FindMember use std::string, but it

also use internal::StrLen to get the string lengtht,
when it call FindMember(StringRef(name)).
Now use GenericValue construct it, then can use the std::string.size.
now it will be faster.

include/rapidjson/document.h

@@ -1169,8 +1169,8 @@ public:
         \return Iterator to member, if it exists.
             Otherwise returns \ref MemberEnd().
     */
-    MemberIterator FindMember(const std::basic_string<Ch>& name) { return FindMember(StringRef(name)); }
-    ConstMemberIterator FindMember(const std::basic_string<Ch>& name) const { return FindMember(StringRef(name)); }
+    MemberIterator FindMember(const std::basic_string<Ch>& name) { return FindMember(GenericValue(StringRef(name))); }
+    ConstMemberIterator FindMember(const std::basic_string<Ch>& name) const { return FindMember(GenericValue(StringRef(name))); }
 #endif
 
     //! Add a member (name-value pair) to the object.