Fix the failing spec in POST /users API

Signed-off-by: Rémy Coutable <remy@rymai.me>

changelogs/unreleased/fix-users-api-500-error.yml

+---
+title: Fix 500 errors when creating a user with identity via API
+merge_request: 8442
+author:

lib/api/users.rb

@@ -91,10 +91,11 @@ module API
         authenticated_as_admin!
         # Filter out params which are used later
-        identity_attrs = params.slice(:provider, :extern_uid)
+        user_params = declared_params(include_missing: false)
+        identity_attrs = user_params.slice(:provider, :extern_uid)
         confirm = params.delete(:confirm)
-        user = User.new(declared_params(include_missing: false))
+        user = User.new(user_params.except(:extern_uid, :provider))
         user.skip_confirmation! unless confirm
         if identity_attrs.any?
@@ -159,11 +160,7 @@ module API
           end
         end
-        # Delete already handled parameters
-        user_params.delete(:extern_uid)
-        user_params.delete(:provider)
-
-        if user.update_attributes(user_params)
+        if user.update_attributes(user_params.except(:extern_uid, :provider))
           present user, with: Entities::UserPublic
         else
           render_validation_error!(user)