FieldType for arrays is now Array

NpgsqlDataReader.GetFieldType() and GetProviderSpecificFieldType() now return Array for arrays.
Previously they returned int[] even for multidimensional arrays, which seems problematic (Array
is at least a superclass of all arrays). Making these methods return the actual dimensioned
type e.g. (int[][]) would be very complicated in sequential mode (part of the column is
already consumed).
Added invariant culture to all numeric conversions