Don't use intermediate OpenSSL::Digest

cc @tmm1

Author: Chris Hunt

lib/html/pipeline/camo_filter.rb

@@ -56,8 +56,7 @@ def asset_proxy_url(url)
 
       # Private: calculate the HMAC digest for a image source URL.
       def asset_url_hash(url)
-        digest = OpenSSL::Digest.new('sha1')
-        OpenSSL::HMAC.hexdigest(digest, asset_proxy_secret_key, url)
+        OpenSSL::HMAC.hexdigest('sha1', asset_proxy_secret_key, url)
       end
 
       # Private: Return true if asset proxy filter should be enabled