UnmarshalJSON in Int now returns an error message when asked to unmarshal from an invalid type. This is in line with the behavior of the built-in sql.NullInt64.
UnmarshalJSON in null.Bool, null.Float, and null.String now return an error message when asked to unmarshal from an invalid type. This is in line with the behavior of the built-in sql library's null values.
Merge pull request #1 from joelkek/master

UnmarshalJSON should return an error for invalid types