Allow headers to be cached along with the response

Garner doesn't support this out of the box. I applied this monkey patch:
artsy/garner#54

config/initializers/garner.rb

@@ -1,8 +1,52 @@
 require "garner"
 require "garner/mixins/mongoid"
+require "garner/mixins/rack"
 
 module Mongoid
   module Document
     include Garner::Mixins::Mongoid::Document
   end
+end
+
+# Monkey patch to cache headers until this is built into Garner
+# https://github.com/artsy/garner/issues/54
+module Garner
+  module Cache
+    class Identity
+      TRACKED_HEADERS = %w{ Link X-Total-Count X-Total-Pages X-Current-Page X-Next-Page X-Previous-Page }
+
+      alias_method :_fetch, :fetch
+      def fetch(&block)
+        if @key_hash[:tracked_grape_headers]
+          result, headers = _fetch do
+            result = yield
+            headers = @ruby_context.header.slice(*TRACKED_HEADERS) if @ruby_context.respond_to?(:header)
+            [result, headers]
+          end
+
+          (headers || {}).each do |key, value|
+            @ruby_context.header(key, value) if @ruby_context.respond_to?(:header)
+          end
+
+          result
+        else
+          _fetch(&block)
+        end
+      end
+
+    end
+  end
+end
+
+module Garner
+  module Mixins
+    module Rack
+
+      alias_method :_garner, :garner
+      def garner(&block)
+        response, headers = _garner.key({ tracked_grape_headers: true }, &block)
+      end
+
+    end
+  end
 end