Make UserExtractor#initialize the same as CE

While this code was backported to CE in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25741, we forgot
to change the `initialize` method in EE to be the same as in CE.

lib/gitlab/user_extractor.rb

@@ -11,7 +11,9 @@ module Gitlab
     USERNAME_REGEXP = User.reference_pattern
 
     def initialize(text)
-      @text = text.is_a?(Array) ? text.join(' ') : text
+      # EE passes an Array to `text` in a few places, so we want to support both
+      # here.
+      @text = Array(text).join(' ')
     end
 
     def users