FIX: mixed case group mentions were not getting highligted in composer

app/controllers/users_controller.rb

@@ -211,7 +211,6 @@ class UsersController < ApplicationController
   def is_local_username
     usernames = params[:usernames]
     usernames = [params[:username]] if usernames.blank?
-    usernames.each(&:downcase!)
 
     groups = Group.where(name: usernames).pluck(:name)
     mentionable_groups =
@@ -223,6 +222,7 @@ class UsersController < ApplicationController
       end
 
     usernames -= groups
+    usernames.each(&:downcase!)
 
     result = User.where(staged: false)
                  .where(username_lower: usernames)

spec/controllers/users_controller_spec.rb

@@ -1516,6 +1516,7 @@ describe UsersController do
   describe ".is_local_username" do
 
     let(:user) { Fabricate(:user) }
+    let(:group) { Fabricate(:group, name: "Discourse") }
 
     it "finds the user" do
       xhr :get, :is_local_username, username: user.username
@@ -1524,6 +1525,13 @@ describe UsersController do
       expect(json["valid"][0]).to eq(user.username)
     end
 
+    it "finds the group" do
+      xhr :get, :is_local_username, username: group.name
+      expect(response).to be_success
+      json = JSON.parse(response.body)
+      expect(json["valid_groups"][0]).to eq(group.name)
+    end
+
     it "supports multiples usernames" do
       xhr :get, :is_local_username, usernames: [user.username, "system"]
       expect(response).to be_success