[Bug Report] el-collapse-transition: overflow not reset when switching to fast

### Element UI version
2.9.0

### OS/Browsers version
Chrome

### Vue version
2.6.10

### Reproduction Link
https://jsfiddle.net/aurelienlt89/mp27wuhb/10/

### Steps to reproduce
Click on the "Try" button

### What is Expected?
The `overflow` should be reset to its original value (`undefined`)

### What is actually happening?
The `overflow` stays to `"hidden"`

This is due to the functions `enter` and `beforeLeave` not checking the state while saving the current `overflow` in `oldOverflow`.
Because of that, `oldOverflow` is set to `"hidden"` when it should be untouched because in transition already.
It is also the case for other saved styles: `padding-top` and `padding-bottom`