Add vertical center alignment into DefaultMonthHeader

[jeongth9446]

I think center vertical alignment is needed in DefaultMonthHeader.

[boguszpawlowski]

@jeongth9446 Sorry for making you wait this much time, I was on vacations. Thank you very much for your contribution, could you just add a screenshot with your change visible to the description of the PR?

[jeongth9446]

Thank you for your interest in my PR.

There will be no problem with the default font size of most devices, but if the font size displayed changes, the alignment may be distorted.

below AS-IS is issue case.

AS-IS

TO-BE (adding verticalAlignment)