Language detection defaults to nil (#3666)

* Default to nil for statuses.language

* Language detection defaults to nil instead of instance UI default

db/migrate/20170609145826_remove_default_language_from_statuses.rb

@@ -0,0 +1,5 @@
+class RemoveDefaultLanguageFromStatuses < ActiveRecord::Migration[5.1]
+  def change
+    change_column :statuses, :language, :string, default: nil, null: true
+  end
+end