Consider using a different diff library

[lestrrat]

I wrote a few PRs so far, and I must say the diff library being used in the test is not quite up to par: For example, it couldn't detect a difference between a payload with and without a trailing comma. And in other situations I got really confusing outputs, so I had to rely on priting t.Logf("%x", ...) to see the hex output and do a diff-by-your-own-eyes.

Would it possible to consider using github.com/pmezard/go-difflib? It seems to be the least broken library out there.