Plug.Conn.Query.encode, creates query strings.

Here's the `encode` complement to the already-implemented `decode` function.

I would have liked to use `URI.encode_query` on the final list of field/value tuples, but that encoding function encodes the `[` and `]` characters, which makes it pretty unreadable, so I just did my own escapes and joins with `&` and `=`.

Always open to suggestions and criticism! :)
There should not be a need for this flatten too.