fix(ui5-daypicker): date selection range light blue indication

[kieransukachevin]

Background

When selecting a date range in the Calendar with selection mode Range, if you select a new start date (and thus have only one date selected), a range of dates have their background color set to light blue indicating they are in the selected date range. This is incorrect since there is only one date selected and therefore no date range yet.

Issue

The problem is that the DayPicker that the Calendar implements has a property called _secondTimestamp that keeps track of the current date that the mouse is hovering (for selection mode Range). When a single date is selected, this allows the DayPicker to update the dates between the selected date and the current date hovered by the mouse (or focused) to have a light blue background color. DayPicker also updates _secondTimestamp to the timestamp property, which is the current selected date, when only one date was previously selected and a new date is selected. But when two dates were previously selected and a new date is selected, _secondTimestamp is not updated to timestamp. Thus, when two dates were previously selected and a new date is selected, dates between the new date and _secondTimestamp which did not update to the new date are marked in light blue.

• Calendar with selection mode Range in test page.
  
• 15 is selected first, then 8. Then, when the new date 21 is selected, dates 9 (which is the value of _secondTimestamp due to 9 being the last date hovered before a second date was chosen) to 20 are light blue.

Change

In order to fix this, _secondTimestamp needs to be updated to the current timestamp property when a new start date is selected. This is done by changing the _updateSecondTimestamp method (called when a new date is selected) to update the _secondTimestamp to the current timestamp when there were previously two dates already selected. Previously, _updateSecondTimestamp only updated _secondTimestamp to the current timestamp when there was previously one date already selected.

• Fixed Calendar with selection mode Range in test page.
  
• 15 is selected first, then 8. Now, when the new date 21 is selected no other dates are light blue.

Fixes #6781

Pull Request Checklist

• Reviewed the Contributing Guidelines
  • Especially the How to Contribute section
• Correct commit message style
  • Bugfixes should generally include a test to cover the issue.