support parsing strings with escaped null character

Expected: parsed_ = eval.parseString("ucase('12345~!@#$%^&*()_+uPpEr{}[];:<>/?\\67890\t \0\b\r\f') == " "'12345~!@#$%^&*()_+UPPER{}[];:<>/?\\67890\t \0\b\r\f'") doesn't throw an exception.
  Actual: it throws std::exception-derived exception with description: "<string>:1.7: Invalid character: '".

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