api/groups: support plain strings for query fields

CHANGELOG.md

@@ -19,7 +19,8 @@ been added.
 
   * The `api::projects::environments::Environment` endpoint uses the correct
     path now.
-  * The `api::projects::members::ProjectMembers` and
+  * The `api::groups::members::GroupMembers`,
+    `api::projects::members::ProjectMembers`, and
     `api::projects::repository::Branches` endpoints now accepts plain strings
     for their `query` fields.
   * The `api::projects::protected_branches::UnprotectBranch` endpoint now

src/api/groups/members/members.rs

@@ -13,6 +13,7 @@ use crate::api::endpoint_prelude::*;
 
 /// Query a members of a group.
 #[derive(Debug, Builder)]
+#[builder(setter(strip_option))]
 pub struct GroupMembers<'a> {
     /// The group to query for membership.
     #[builder(setter(into))]