Fix typo in at_path

[ismagilli]

What does this change do? Using a static code analyzer, a suspicious place was discovered:

if (c == ']')
{
    pos++;
    break;
}
else if TOML_UNLIKELY(c == '.' || c == '[')
//                    ^^^^^^^^
    break;
else if (c == '\t' || c == '.')
//                    ^^^^^^^^
    pos++;
else
    return false;

Obviously, a mistake was made in one of the places. It is easy to see that in the second case, whitespace characters are checked, so there should be a space (' ') instead of a dot ('.'). This PR fixing bug, adds a test.

Is it related to an exisiting bug report or feature request? No, I created a PR without creating an issue

Pre-merge checklist

• [x] I've read CONTRIBUTING.md
• [x] I've rebased my changes against the current HEAD of origin/master (if necessary)
• [x] I've added new test cases to verify my change
• [x] I've regenerated toml.hpp (how-to)
• [x] I've updated any affected documentation
• [x] I've rebuilt and run the tests with at least one of:
  • [ ] Clang 6 or higher
  • [x] GCC 7 or higher
  • [ ] MSVC 19.20 (Visual Studio 2019) or higher
• [x] I've added my name to the list of contributors in README.md

[marzer]

Nice find, thanks!