Update naming for header methods on requests

The putHeaders/withHeaders methods are confusing for multiple people.

Let’s deprecate them and add addHeaders and replaceHeaders methods respectively. Those names have immediately-obvious semantics.

[rossabaker]

There are a lot of ways to do this, so let’s try to be a bit more rigorous in our definitions.

1. Some headers are recurring (e.g., Accept), and some are singletons (e.g., Content-Length). A recurring header can have multiple instances. A singleton header only sensibly has one.
2. Every recurring header can be combined into a single header by joining their values with a comma, except Set-Cookie, because our forebears hated us and wanted us to be unhappy.
3. The semantics depend on the key.

I’m going to intentionally avoid our existing names for things here, because I can’t remember what the damn things do. But there are several possible implementations of (a: Headers, b: Headers) => Headers. Which ones do we want, and what should they be called?

1. Append a to b, regardless of semantics.
2. Combine headers by key in a and b, joining duplicates with a comma, regardless of semantics.
3. Replace singletons from a with b. Append recurring headers from b to b. Keep other headers from a.
4. Replace singletons from a with b. Combine recurring headers by key in a and b. Keep other headers from a.
5. Replace all headers from a with b. Keep other headers from a.
6. Return b
7. ???

[rossabaker]

I’m not aware of any active effort to change them from 0.23. Is there something you’d still like to see?