Also fix typo in the message flags enum

I wasn't sure if this was going to work but the documentation says:

> The "omitempty" option specifies that the field should be omitted from the encoding if the field has an empty value, defined as false, 0, a nil pointer, a nil interface value, and any empty array, slice, map, or string.

Therefore, if Flags is unmodified it will be omitted.

I think the null value here may be triggering the "Cannot edit a message authored by another user"

Stops the gofmt error