Fix small value Unicode JSON parse error

this fixes this error we've getting back from the Sendgrid SMTP api:
code:400 error:<nil> body:{"message": "error", "errors": ["JSON in x-smtpapi could not be parsed"]}
...
our response contains mangled Unicode characters:
{"body":"\u003cp\u003eYouâ€™re a winner!"}
...
adding the 4 character pad allows fmt.Sprintf to format the encoding correctly