How do I add a query parameter?

How do I use URIs.jl to add a query parameter to a URL that _might already have query parameters_?

e.g.

```julia
URIs.magicfunction(URI("http://hello.com/page?x=123", Dict("y" => "no")) ==
	URI("http://hello.com/page?x=123&y=no")

URIs.magicfunction(URI("http://hello.com/page", Dict("y" => "no")) ==
	URI("http://hello.com/page?y=no")
```