Package Instance

Http4s improperly parses User-Agent and Server headers
### Impact

The `User-Agent` and `Server` header parsers are susceptible to a fatal error on certain inputs.  In http4s, modeled headers are lazily parsed, so this only applies to services that explicitly request these typed headers. 

#### v0.21.x

```scala
val unsafe: Option[`User-Agent`] = req.headers.get(`User-Agent`)
```

#### v0.22.x, v0.23.x, v1.x

```scala
val unsafe: Option[`User-Agent`] = req.headers.get[`User-Agent`]
val alsoUnsafe: Option[`Server`] = req.headers.get[Server]
```

### Patches

Fixes are released in 0.21.34, 0.22.15, 0.23.17, and 1.0.0-M38.

### Workarounds

#### Use the weakly typed header interface

##### v0.21.x

```scala
val safe: Option[Header] = req.headers.get("User-Agent".ci)
// but don't do this
val unsafe = header.map(_.parsed) 
```

##### v0.22.x, v0.23.x, v1.x

```scala
val safe: Option[Header] = req.headers.get(ci"User-Agent")
```