Is it intentional that the default number format formatCode is "GENERAL" instead of "General"

[flummer]

When I open a file created with XLSXWriter in Numbers on macOS, I get a warning on the header fields that states:

The data format couldn't be imported
Original format: GENERAL

I don't get this error in Excel on Windows or Libre Office on macOS

When I change it to "General" (5 places in xlsxwriter.class.php including one in a comment) the files open without warnings in all the above tested applications.

From ECMA 376 it seems to me that the formatCode should be "General", but maybe I'm missing something.

Should I make a PR to change this?

[maksimovic]

@flummer I've ported your changes to https://github.com/maksimovic/PHP_XLSXWriter/releases/tag/1.0.3