[feature request] `get_one`, `get_one_mut` on `HeadersMap`

I'll write this like a proposal:

Motivation

I want to be able to get a header from a HeadersMap, and also assert that there is exactly one header with the given name. Currently I do what I'm proposing here, but in my client library. I think this is functionality that would be useful to multiple consumers of http and so maybe could be in the library itself.

Proposal

Introduce functions get_one and get_one_mut on HeadersMap, with the following implementation (or equivalent, more performant impl):

impl<T> HeaderMap<T> {
    pub fn get_one<K>(&self, key: K) -> Result<&T, GetOneError> 
        where K: AsHeaderName
    {
        let mut iter = self.get_all(key);
        let value = match iter.next() {
             Some(v) => v,
             None => return Err(GetOneError::Missing)
        };
        if let Some(_) = iter.next() {
            Err(GetOneError::Multiple);
        } else {
            Ok(value)
        }
    }
}

with a similar impl for get_one_mut, and the implied GetOneError std::error::Error.

You could probably get the count for Multiple without hitting the happy path, if that was considered worthwhile.

Start by locating HeadersMap and its existing get_all API, then review nearby tests and error types. Implement the proposed get_one and get_one_mut behavior so missing and multiple values return distinct errors while exactly one value returns successfully, and verify both shared and mutable access.