fix: empty path segments produced by build_url()

[lazulit3]

When testing my middleware in another project I noticed that rustify will sometimes produce extraneous // in the Uri produced by http::build_url().

This is due to some unhandled cases where base path, trailing slashes, and path with leading slashes may result in adding empty path segments to the joined path.

Testing & Failure Cases

This PR adds 9 different test cases. Prior to the fix, 3 were failing.

You can checkout f7ab901 and cargo test to reproduce.

Example

This is an example similar to how I stumbled on this:

    use rustify::Endpoint;
    use rustify_derive::Endpoint;

    #[derive(Endpoint)]
    #[endpoint(path = "/endpoint")]
    struct ExampleEndpoint {
        #[endpoint(query)]
        hello: String,
    }

    #[test]
    fn rustify_build_url_extra_slash_example() {
        let endpoint = ExampleEndpoint {
            hello: "world".into(),
        };
        let expected = "https://192.0.2.2/api/endpoint?hello=world";

        let request_uri = endpoint.request("https://192.0.2.2/api").unwrap();

        assert_eq!(request_uri.uri(), expected);
    }

assertion `left == right` failed
  left: https://192.0.2.2/api//endpoint?hello=world
 right: "https://192.0.2.2/api/endpoint?hello=world"

This issue doesn't really impact actual HTTP requesting / library usage, but may surprise developers trying to write tests. :)

Fix

build_url() is adjust to better avoid empty path segments by:

• popping off empty segment on the base URL path (for cases where it had a trailing slash)
• stripping the leading / from the endpoint/relative path (to avoid adding an empty path segment)