Internal error: entered unreachable code: Got unexpected tag type

Hi, I found an input that causes an internal error when calling `rewrite_str`. I am attaching a small program that reproduces the issue (both on 0.3.1 and latest `master`).
```rust
fn main() {
let _ = lol_html::rewrite_str(r"

Start by running the supplied Rust program with lol_html::rewrite_str and Settings::default() to reproduce the panic. Inspect parser/tree_builder_simulator/mod.rs at line 214 and trace the unexpected tag type; done means this input returns Err(_) rather than triggering an internal panic.

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