include example of header encodings

[handrews]

Apparently I did not include an example of allow (which for some reason I really thought I had). There's also not a direct example of it in the referenced draft spec for encoding headers in JSON. This has led to at least one instance of someone considering Allow: HEAD, GET, POST to be encoded as "allow": [["HEAD", "GET", "POST"]], treating compound values within and among individual header lines differently. That was not the intention.

[handrews]

Also reference the part here about how repeated headers and multiple comma-separated values have the same semantics, which is why Allow does not end up with a double-array. The headers:

Allow: HEAD, GET
Allow: POST

are equivalent to

Allow: HEAD, GET, POST

https://tools.ietf.org/html/rfc7230#section-3.2.2

[handrews]

Merged #675