Should store headers and query params as a list instead of a map

[gmethvin]

Storing as a map causes re-ordering of query params which might be undesirable in some cases. It would be fine to use a Seq[(String, String)].

See playframework/playframework#6884, specifically https://github.com/playframework/playframework/pull/6884#issuecomment-272674376

[richdougherty]

Reopening because we only have a partial fix in #239.