[Bug Report] offsetFromStart falsy w/ getStartDateOfMonth/firstDayOfMonth on the same row and 1 < firstDayOfWeek < 4 in data-table component

### Element UI version
2.14.0

### OS/Browsers version
all

### Vue version
2.6.12

### Reproduction Link
https://jsfiddle.net/5bowryx3/

### Steps to reproduce
Select daterange 09/01/2020 - 09/30/2020

### What is Expected?
Receive [ "2020-08-31T22:00:00.000Z", "2020-09-29T22:00:00.000Z" ] in model

### What is actually happening?
Receive [ "2020-09-07T22:00:00.000Z", "2020-10-06T22:00:00.000Z" ] in model