FIX: Check group names when checking username availability

app/models/user.rb

@@ -208,7 +208,8 @@ class User < ActiveRecord::Base
   def self.username_available?(username, email = nil)
     lower = username.downcase
     return false if reserved_username?(lower)
-    return true  if !User.exists?(username_lower: lower)
+    return true  if User.exec_sql(User::USERNAME_EXISTS_SQL, username: lower).count == 0
+
     # staged users can use the same username since they will take over the account
     email.present? && User.joins(:user_emails).exists?(staged: true, username_lower: lower, user_emails: { primary: true, email: email })
   end

spec/models/user_spec.rb

@@ -554,6 +554,11 @@ describe User do
       user = Fabricate(:user, email: "bar@foo.com")
       expect(User.username_available?(user.username, user.primary_email.email)).to eq(false)
     end
+
+    it 'returns false when a username equals an existing group name' do
+      Fabricate(:group, name: 'foo')
+      expect(User.username_available?('Foo')).to eq(false)
+    end
   end
 
   describe '.reserved_username?' do