signed tinyint fields are read as unsigned

```
create table test (
  field1 tinyint not null
);

insert into test ( field1 ) values ( -1 );

select field1 from test; -- returns 255
```
The issue is that tinyint fields are always considered unsigned:
https://github.com/andrenth/ocaml-mariadb/blob/59b6eec1a2880747d92d3edc35c6cf20e829d516/lib/field.ml#L77