Fix panics in `Headers::try_insert` and `Headers::try_append` (#3157)

The `try_insert` and `try_append` methods on `Headers` would panic when
passing a non-ASCII value for the key, or an invalid UTF-8 value for the
value. This was due to them using the http crate's `from_static` method,
which panics on these invalid values. This PR makes them always use
`try_from` instead.

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