Previously if we encountered a keyword other than `fill` when parsing
`<border-image-slice` we would return a nullptr.
This could cause issues when we parse `<border-image-slice>` as part of
parsing `border-image`, for example `border-image: 100% none` would fail
as we would try parse `none` as part of the `<border-image-slice>`
instead of `<border-image-source>`.
This change makes it so that we don't consume the token and leave it to
be parsed as part of the next section of the grammar.
