Adding headers with multiple casings of the same field name

WhatWG fetch specifies that if there is an existing header Foo: 42, and you go to append foo: 69, the lower-case header name is first corrected to the casing of the existing Foo: https://fetch.spec.whatwg.org/#concept-header-list-append

I understand that one goal of wasi-http is to be implementable in terms of whatwg fetch. This constrains us, in that we can't support both the contract of append and copy-all:

   /// Append a value for a name. Does not change or delete any existing
    /// values for that name.
    ///
    /// Fails with `header-error.immutable` if the `fields` are immutable.
    append: func(name: field-name, value: field-value) -> result<_, header-error>;
    /// Retrieve the full set of names and values in the Fields. Like the
    /// constructor, the list represents each name-value pair.
    ///
    /// The outer list represents each name-value pair in the Fields. Names
    /// which have multiple values are represented by multiple entries in this
    /// list with the same name.
    ///
    /// The names and values are always returned in the original casing and in
    /// the order in which they will be serialized for transport.
    copy-all: func() -> list<tuple<field-name, field-value>>;

I suggest that we adopt to make set and append operations first adapt the case of the field-name to any existing field-name.

Start with the wasi-http header append, set, and copy-all definitions quoted in the issue, then compare their casing behavior with the linked WHATWG Fetch header-list append specification. Determine whether the interface can preserve both contracts; done means the specification and operations have an agreed, consistent casing behavior.