[webmention] Issue: section 3.1.2 charset marked as i18n

r12a has just labeled an issue for https://github.com/w3c/webmention 
as "i18n":

== section 3.1.2 charset ==
https://www.w3.org/TR/webmention/#sender-notifies-receiver

Section 3.1.2 describes the submission of the `source` and `target` 
URLs in the `x-www-form-urlencoded` format. There is no mention of 
character encoding, which normally is an important concern for this 
format. However, since the strings in question are URLs, they are 
presumably already "URL encoded" using the character encoding 
recognized by the host server. I'm raising this issue to point out 
that the charset issue works in this case. However, if fields were 
added to the payload in a future revision, the charset might become 
important.

See https://github.com/w3c/webmention/issues/56