prefixPath in gatsby-config prevents creation of sitemap.xml and proper RSS feed

[KnowIsPow]

Issue Summary
XML sitemaps aren't created, and RSS feeds to not load properly when using prefix paths.

To Reproduce

1. add pathPrefix to gatsby-config file
2. add siteURL to siteConfig.js (with prefix added but without trailing slash, eg www.example.com/pathPrefix)
3. run gatsby build --prefix-paths

[Romstar]

Experiencing the same thing, I'll post back here if I find a fix.