Set the Buffer's timestamp when setting a tag

lib/rackstash/buffer.rb

@@ -252,6 +252,7 @@ module Rackstash
     # @return [Fields::Tags] the resolved tags which are set on the buffer.
     #   All strings are frozen.
     def tag(*new_tags, scope: nil)
+      timestamp
       tags.merge!(new_tags, scope: scope)
     end
 

spec/rackstash/buffer_spec.rb

@@ -429,6 +429,11 @@ describe Rackstash::Buffer do
       expect(buffer.tags).to contain_exactly('tag')
     end
 
+    it 'sets the timestamp' do
+      expect(buffer).to receive(:timestamp)
+      buffer.tag('hello')
+    end
+
     describe 'when passing procs' do
       let(:struct) {
         Struct.new(:value) do