The article states GitHub gists add the correct content-type header based on file extension which is untrue.

  curl -i https://gist.githubusercontent.com/mikeknoop/e7b3c526738b66950eb4/raw/1d46d432ed380abc986cf15028221318b836395b/text.xml
  // other headers...
  Content-Type: text/plain
  // headers and body...

There is a great service call rawgit[1] that does actually add the correct headers.

[1] https://rawgit.com/