Fix the string replace

[thirdwing]

The CRAN version:

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
String replace() {
  String foo("abba");
  return foo.replace_first("ba", "BA");
}

/*
> Rcpp::sourceCpp("test.cpp")
> replace()
[1] "BAba"
*/

After fix:

> Rcpp::sourceCpp("test.cpp")
> replace()
[1] "abBA"

@kevinushey

[eddelbuettel]

Looks good first glance.

[kevinushey]

Looks good to me as well -- I kind of wish we could get away without copying the C strings into std::strings, but I'm fine with the PR as-is.

[kevinushey]

Thanks for doing the legwork @thirdwing!