Fix `dn?` for a UID with an escaped equal sign

lib/gitlab/ldap/person.rb

@@ -50,7 +50,7 @@ module Gitlab
       # An empty string is technically a valid DN (null DN), although we should
       # never need to worry about that.
       def self.dn?(uid_or_dn)
-        uid_or_dn.blank? || uid_or_dn.include?('=')
+        uid_or_dn.blank? || !!uid_or_dn.match(/(?<!\\)=/)
       end
 
       # Returns the UID in a normalized form.

spec/lib/gitlab/ldap/person_spec.rb

@@ -100,12 +100,13 @@ describe Gitlab::LDAP::Person do
     where(:test_description, :given, :expected) do
       'given a DN with a single RDN'                     | 'uid=John C. Smith'                                 | true
       'given a DN with multiple RDNs'                    | 'uid=John C. Smith,ou=People,dc=example,dc=com'     | true
-      'given a UID'                                      | 'John C. Smith'                                     | false
       'given a DN with a single RDN with excess spaces'  | ' uid=John C. Smith   '                             | true
       'given a DN with multiple RDNs with excess spaces' | '  uid=John C. Smith,ou=People,dc=example,dc=com  ' | true
-      'given a UID with excess spaces'                   | '   John C. Smith '                                 | false
       'given a DN with an escaped equal sign'            | 'uid=John C. Smith,ou=People\\='                    | true
       'given a DN with an equal sign in escaped hex'     | 'uid=John C. Smith,ou=People\\3D'                   | true
+      'given a UID'                                      | 'John C. Smith'                                     | false
+      'given a UID with excess spaces'                   | '   John C. Smith '                                 | false
+      'given a UID with an escaped equal sign'           | 'John C. \\= Smith'                                 | false
     end
 
     with_them do