If you do the following:

headers.append("accept", "text/plain")
headers.append("Accept", "application/x-www-form-urlencoded")

I assume all of these should return an array of two items:

headers.getAll("accept");
headers.getAll("Accept");
headers.getAll("ACCEPT");

But if we have to serialize out, does it matter what form of the header case should be used?

I propose allowing the browser to select a single case for output serialization, iteration, etc. Preserving case seems relatively unimportant and would require memory/complexity.