[ShadyDOM] Edge slots not working if created with uppercase name

### Description
Slots are not replaced if created with uppercase names on edge.
The issue seems to be that `localName` is compared against `slot`

While i suspect this is not a common occurrence it gave me quite a bit of debugging.

#### Live Demo
https://codepen.io/FireNeslo/pen/ZZLBvV?editors=1010

#### Steps to Reproduce

1. Create `my-element`
2. Create `SLOT` (uppercase)
3. Create `span`
4. Attach shadow to `my-element`
5. Append `SLOT` to `my-element.shadowRoot`
6. Append `span` to `my-element`
7. Append `my-element` to `body`

#### Expected Results
Expected `SLOT` to be replaced with `span`
#### Actual Results
`SLOT` is still there
### Browsers Affected

- [ ] Chrome
- [ ] Firefox
- [x] Edge
- [ ] Safari 9
- [ ] Safari 8
- [ ] IE 11

### Versions
- webcomponents: v2.2.7