Fix invalid continuation byte for webapi

I get this error when using API:
{"result":"error", "message": "error while composing output: "utf8 : terça 9:00  : error 3 : error 4 : invalid continuation byte"}

There's a special char "TERÇA"

Don't know why this happens. I had a special date style and even after change it to "yyyy-mm-dd" this error still happens. Don't know why the api builder is still using the translated week day. Also timezone is set to "network"

This error only happens when missing/future has "terça" - Tuesday and "sábado" - saturday weekdays. The other week days there's no special char.
Merge pull request #934 from fernandog/patch-6

Fix invalid continuation byte for webapi