OIDC: make response_mode optional

Fractal omits the `response_mode` field when in an auth flow (its value must be
the literal "S256", so it's mainly here for OIDC compliance I guess). Accepting
this lets it proceed to the next authentication step.

src/web/oidc/consent.rs

@@ -1,4 +1,3 @@
-use std::borrow::Cow;
 use askama::Template;
 use axum::http::StatusCode;
 use oxide_auth::frontends::simple::request::Body;
@@ -27,11 +26,7 @@ pub fn oidc_consent_form(hostname: &str, query: &AuthorizationQuery) -> OidcResp
 
 /// Render the html contents of the user consent page.
 fn consent_page(hostname: &str, query: &AuthorizationQuery, route: &str, nonce: &str) -> String {
-	let response_mode = &query.response_mode.clone()
-		.unwrap_or_else(|| match query.redirect_uri.scheme() {
-			| "https" => Cow::Borrowed("fragment"),
-			| _ => Cow::Borrowed("query")
-		});
+	let response_mode = &query.response_mode.clone().unwrap_or("fragment".to_string());
 	let template = ConsentPageTemplate {
 		nonce,
 		hostname,

src/web/oidc/login.rs

@@ -62,13 +62,9 @@ impl TryFrom<OidcRequest> for LoginQuery {
 		let Ok(redirect_uri) = Url::from_str(&redirect_uri) else {
 			return Err(LoginError("invalid field: redirect_uri".to_owned()));
 		};
-		// response_mode is not strictly needed : its value defaults to "fragment"
+		// response_mode is not strictly needed : it must be the literal "fragment"
 		// when over https. It's required by the spec but Fractal doesn't provide it.
-		let response_mode = body.unique_value("response_mode")
-			.unwrap_or_else(|| match redirect_uri.scheme() {
-				| "https" => Cow::Borrowed("fragment"),
-				| _ => Cow::Borrowed("query")
-			});
+		let response_mode = body.unique_value("response_mode").unwrap_or(Cow::Borrowed("fragment"));
 		let client_secret = body.unique_value("client_secret").map(|s| s.to_string());
 
 		Ok(Self {
@@ -108,12 +104,7 @@ pub fn oidc_login_form(hostname: &str, query: &AuthorizationQuery) -> OidcRespon
 
 /// Render the html contents of the login page.
 fn login_page(hostname: &str, query: &AuthorizationQuery, route: &str, nonce: &str) -> String {
-	let response_mode = &query.response_mode
-		.clone()
-		.unwrap_or_else(|| match query.redirect_uri.scheme() {
-			| "https" => "fragment".to_string(),
-			| _ => "query".to_string()
-		});
+	let response_mode = &query.response_mode.clone().unwrap_or("fragment".to_string());
 	let template = LoginPageTemplate {
 		nonce,
 		hostname,