Correction to MDI handling.

Thanks to @bjlittle reporting some unexpected behaviour, further investigation into the handling of missing values by the ECMWF GRIB API has highlighted a misunderstanding:
 - Keys with signed values return 2**32-1, and **not** -(2**31-1) as previously thought.

Happily, this makes things simpler. \o/
How come we don't get a problem here ?
Old test was == -(2 ** 31 - 1)
New test is == 2 ** 32 - 1
???