add gzip-bomb test case

[sknebel]

(Idea in IRC http://indiewebcamp.com/irc/2016-04-20/line/1461155096489 )

Webmention endpoints that fetch source URLs with Accept-Encoding: gzip or deflate might not properly implement size limits: If they fetch up to e.g. 5 MB of compressed data, this can worst case expand to 5 GB.