[RTClib] (Bugfix) Write month correctly to DS3231 (#3749)

[tonhuisman]

After a report in the forum, then reported in the issue tracker.

Resolves #3749

[TD-er]

Wasn't this something about the century bit also being stored in the month field?

[TD-er]

Yep, looks like a messy copy/paste from me.

[uzi18]

@tonhuisman in fact it should be more like this:

RTCWireBus->_I2C_WRITE(bin2bcd(dt.month()) &0x7f | (dt.year()>=2000 ? 0x80 : 0x00)); // Need to add the century bit

[TD-er]

@uzi18 I just realized it myself, so I went back to this PR to mention something similar.
I think in practice the value can simply have 0x80 added, as the year will always be past 2000 now... I think ;)

[uzi18]

Sure, it is :)