fix: text that looks like an html tag but is not causes [$sanitize:badparse] error

The provided unit test fails with this error.

```
Error: [$sanitize:badparse] The sanitizer was unable to parse the following block of html: <
nonEndingTag href <nonEndingTag href
```

The provided change fixes it.

Can you please backport the fix into v1.2.x as this is a show stopper for us. Thanks
shouldn't we convert the `<` to `&lt;` in this case?