FIX

in User#authenticate:
* make search by username case sensitive as in prod there already are
  usernames diffing only in case
* use guard clause instead of nesting

app/models/user.rb

@@ -449,7 +449,9 @@ class User < ActiveRecord::Base
   end
 
   def self.authenticate(login)
-    if (user = where("LOWER(username) = LOWER(?)", login[:username]).first)
+    user = where('username = ?', login[:username]).first
+    return nil unless user
+
     begin
       case user.password_hash
       when User::PASSWORD_SCRYPT
@@ -480,8 +482,6 @@ class User < ActiveRecord::Base
     #  return nil
     end
   end
-    return nil
-  end
 
   def self.get(id)
     id ? User.find(id) : ""