DayPickerInput doesn't call dayPickerProps.onMonthChange

[ah-adarlow]

DayPickerInput overrides the onMonthChange handler passed in through dayPickerProps and doesn't call it when the month changes.

See DayPickerInput.js line 446.

https://codesandbox.io/s/km00lz2vq3

[gpbl]

Confirmed! Someone may want to fix it :)

• this line should become:

- onMonthChange={month => this.setState({ month })}
+ onMonthChange={this.handleMonthChange}

• handleMonthChange should setState({month}) and use as callback props.dayPickerProps.onMonthChange –if it exists– with the same args
• must add a test for handleMonthChange calling onMonthChange