Wrapped encoding for problematic fields

[jricher]

Some fields, like Set-Cookie, have an internal syntax that allows unquoted commas and messes things up when multiple lines are combined for the signature base, leading to the potential case where two semantically different inputs have the same signature base. This means that:

Set-Cookie: that, happens
Set-Cookie: to, contain, a, comma

And:

Set-Cookie: that, happens, to, contain, a, comma

Both produce the same signature input line:

"set-cookie": that, happens, to, contain, a, comma

Even though the two-line and single-line versions are processed differently by the application.

To counter this, we could have a distinct encoding flag that wraps the field values, similar to sf. This can be used to protect problematic fields like Set-Cookie so that we get something like this for the multiple line version:

"set-cookie";b64: :dGhhdCwgaGFwcGVucw==:, :dG8sIGNvbnRhaW4sIGEsIGNvbW1h:

But you'd get this for the single-line version:

"set-cookie";b64: :dGhhdCwgaGFwcGVucywgdG8sIGNvbnRhaW4sIGEsIGNvbW1h:

The background of this has been discussed in #2143 and #1183