obj -> string for FsHttpUrl.additionalQueryParams

[ghost]

This PR resolves issue 102 based on the following assumptions:

• A preferred implementation would use (string * string) list instead of (string * obj) list for both the query and formUrlEncoded functions.
• Changing the type of FsHttpUrl.additionalQueryParams does not result in tests failing. This includes the test cases in src/Tests/UrlsAndQuery.fs. The tests pass string * string instead of string * obj
• Asking users to perform value formatting for the call decouples the 'stringification' for use with FsHttp from any other uses outside the bounds of FsHttp.

[dawedawe]

Thanks a lot @RonaldTM9999 , we'll have a second look in the next couple of days and make a new major release with it soon.