Add a Feeds20210630 helper with upload_feed_document

This method has previously been mentioned in the README, but was not
defined. This commit defines the method, with a slight change to take
the user requested content-type, which must match due to the signed
headers.

README.md

@@ -147,7 +147,7 @@ upload_url = response.parse["url"]
 
 # Upload the feed content to the provided URL
 feed_content = File.read("inventory_update.xml")
-api.upload_feed_document(upload_url, feed_content)
+api.upload_feed_document(upload_url, feed_content, "text/xml; charset=UTF-8")
 
 # Create the feed
 response = api.create_feed(

lib/peddler/api/feeds_2021_06_30.rb

@@ -2,6 +2,8 @@
 
 require "peddler/api"
 
+require "peddler/helpers/feeds_2021_06_30"
+
 module Peddler
   class << self
     def feeds_2021_06_30(...)
@@ -14,6 +16,8 @@ class API
     #
     # The Selling Partner API for Feeds lets you upload data to Amazon on behalf of a selling partner.
     class Feeds20210630 < API
+      include Peddler::Helpers::Feeds20210630
+
       # Returns feed details for the feeds that match the filters that you specify.
       #
       # @note This operation can make a static sandbox call.

lib/peddler/helpers/feeds_2021_06_30.rb

@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+module Peddler
+  module Helpers
+    module Feeds20210630
+      # Uploads feed_content to a signed upload_url previously provided by
+      # create_feed_document. The upload_url is signed, the Host and content-type
+      # headers must match the signing.
+      # @param upload_url [String] The signed url from the `create_feed_document` response.
+      # @param feed_content [String] The body of the content to upload.
+      # @param content_type [String] The content type of the upload,
+      # this must match the content-type requested in `create_feed_document`
+      # @return [HTTP::Response] The API response
+      def upload_feed_document(upload_url, feed_content, content_type)
+        response = HTTP.headers(
+          "Host" => URI.parse(upload_url).host,
+          "content-type" => content_type,
+        ).send(:put, upload_url, body: feed_content)
+
+        if response.status.client_error?
+          error = Error.build(response)
+          raise error if error
+        end
+
+        response
+      end
+    end
+  end
+end