calendar.visibleDates().monthDates.first?.date and calendar.visibleDates().monthDates.last?.date always coming with a -1 offset in didScrollToDateSegmentWith

* [x] I've read, understood, and done my best to follow the [*CONTRIBUTING guidelines](https://github.com/jjatie/Charts/blob/master/CONTRIBUTING.md).

## What did you do?

Im trying to get first and last date of the month when user scrolls. I'm using didScrollToDateSegmentWith delegate method.

```
func calendar(_ calendar: JTACMonthView, didScrollToDateSegmentWith visibleDates: DateSegmentInfo) {

if let startDate = calendar.visibleDates().monthDates.first?.date, let endDate = calendar.visibleDates().monthDates.last?.date {

getData(startDate: startDate, endDate: endDate)

}
}
```

## What did you expect to happen?

I expected to get startDate and endDate of the corresponding month.

Eg:
**After scrolling to March 2020**
startDate 2020-03-01, endDate 2020-03-31

**After scrolling to April 2020**
startDate 2020-04-01 endDate 2020-04-30

## What happened instead?

Instead I always get startDate and endDate with -1 offset.

**When I set debug points to see startDate and endDate after scrolling to March 2020**
startDate 2020-02-29, endDate 2020-03-30

**When I move to April 2020**
startDate 2020-03-31 endDate 2020-04-29

## Charts Environment

**Charts version/Branch/Commit Number:**
**Xcode version:** Version 11.4.1
**Swift version:** Swift 5
**Platform(s) running Charts:** iOS 13.4.1
**macOS version running Xcode:** 10.15.4

## Demo Project

ℹ Please link to or upload a project we can download that reproduces the issue.