Revisit include_sundays param behavior

[arkid15r]

At least from the perspective of my company, we are only interested in holidays other than Sundays (if we wanted Sundays we could very easily get them without this package). I find it confusing that Sundays are included for Sweden but not for other countries, because for all practical purposes, there is nothing special about Sundays in Sweden (apart from maybe some constitutional definition that bears no practical implication). Also, it caused a bug for us when Sundays coincided with other holidays and the name of the holiday changed, e.g. "Julafton; Söndag".

So regardless of if we want to use include_weekends or include_sundays, I think the default should be False to make it consistent with other countries. But then again, I don't know in which ways others use this package.

Based on #2072

Also related to #1654