This adds a new query= argument for HTTP requests that can be used to

provide additional query arguments. These will be combined with any

query arguments already in the URL, meaning they'll work fine when

combined with any URLs coming from payloads. It also makes it easy for

hosting service implementations to add additional querying or payload

filtering options.
When building the query arguments for a URL, all query argument names

are listed in alphabetical order, rather than dictionary iteration

order. This guarantees a stable URL across all versions of Python.

Unit tests pass.