Rasql_Cannot encode 0D coverage in JSON format

[Bang Pham Huu]

When encoding a coverage in 0D (e.g: slice from both axes of a 2D coverage) in JSON, it will have error

rasql -q 'SELECT encode(c[0,711], "application/json" ) FROM test_mean_summer_airtemp AS c' --out string
rasql: rasdaman query tool v1.0, rasdaman v9.3.2.
opening database RASBASE at localhost:7001...ok
Executing retrieval query...rasdaman error 380: Execution error 380 in line 1, column 8, near token encode: Data type can not be converted to selected data exchange format.
aborting transaction...ok
rasql done.

for encode 1D or 2D it works well, so it needs to be fixed and add test case in system test.