`Curl#parse_curl_response` only includes the last instance of a given
header in its `:headers` hash (replicating pre-existing behavior).
This is a problem for headers like `Set-Cookie`, which can appear more
than once in a response.
This commit addresses the issue by collecting duplicate headers into
an array instead. Headers that only appear once in the response will
still have a string value but headers that appear more than once will
be an array of strings. Whenever headers from `#parse_curl_response`
are used (directly or indirectly), it's important to conditionally
handle the expected types.
