Optimize decode using if-else(O(1)) instead of std::string.find(O(n))

[ReneNyffenegger]

I merged your solution with someone else's solution that tackled the same problem:

https://github.com/ReneNyffenegger/cpp-base64/commit/e6151da60e9a42be59a3cf9fa39edcb1976fe3a7#diff-1e14a5b7a71397146d6e1f49be9462ff