allow http_equiv attribute in meta tags

looks like #23 was incomplete, we should add this and do a patch version, 0.4.1

```
---- site::it_builds_the_site stdout ----
ERROR: <meta http_equiv="Permissions-Policy"> failed to parse attribute value: Matching variant not found
ERROR: rebuild with nightly to print source location
thread 'site::it_builds_the_site' panicked at 'failed to parse string literal', src/site/head.rs:59:9
```