This patch adds code to only parse the HTTP response body as JSON if the
content-type header advertises the content as JSON. In my case, the
directory server was unavailable: it returned a 503 HTTP response code
with an HTML document, and the only thing lego reported was:
2016/04/04 19:12:56 Could not create client: get directory at 'https://acme-v01.api.letsencrypt.org/directory': invalid character '<' looking for beginning of value
This was caused by trying to parse the document body (HTML) as JSON,
without looking at the content-type header and returning the JSON parse
error.
