daysInMonth() returns 1 for april through october when using timezone

**Describe the bug**
```ts
dayjs(new Date(2020, 3, 1)).tz().daysInMonth() // returns 30
dayjs(new Date(2020, 3, 1)).tz('America/New_York').daysInMonth() // returns 1
```
https://stackblitz.com/edit/typescript-tqxa8b?file=index.ts

**Information**
- dayjs 1.9.7
- windows
- chrome 87

Start by running the linked StackBlitz reproduction with Day.js 1.9.7 and compare the two daysInMonth() calls. Trace the timezone-related path used by daysInMonth(); done means the America/New_York call returns 30 for April 2020 while the non-timezone call remains correct.

Written by the indexing model from the issue text.