Add forgotten AtomSerializer spec (#2047)

spec/lib/atom_serializer_spec.rb

+require 'rails_helper'
+
+RSpec.describe AtomSerializer do
+  describe '#author' do
+    it 'returns dumpable XML with emojis' do
+      account = Fabricate(:account, display_name: '💩')
+      xml     = AtomSerializer.render(AtomSerializer.new.author(account))
+
+      expect(xml).to be_a String
+      expect(xml).to match(/<poco:displayName>💩<\/poco:displayName>/)
+    end
+
+    it 'returns dumpable XML with invalid characters like \b and \v' do
+      account = Fabricate(:account, display_name: "im l33t\b haxo\b\vr")
+      xml     = AtomSerializer.render(AtomSerializer.new.author(account))
+
+      expect(xml).to be_a String
+      expect(xml).to match(/<poco:displayName>im l33t haxor<\/poco:displayName>/)
+    end
+  end
+end