optimize FindMember when use std::string

[fuzhufang]

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.

[coveralls]

Coverage remained the same at 100.0% when pulling b67ff2f on fuzhufang:master into 99ba17b on miloyip:master.

[miloyip]

Thanks