Anchor links containing special characters not being stripped

When I define a section containing special characters, for example:
```
## api/path/to/resource/<resource_id>/
```

The preview doesn't strip the slashes and <>, so the anchor link becomes:
```
#api%2Fpath%2Fto%2Fresource%2F<resource_id>%2F
```

But on GitHub the actual anchor link would be:
```
#apipathtoresourceresource_id
```