Various API bugfixes

[TalZaccai]

trafficstars

• [x] TTL / PTTL should return -2 when key does not exist
• [x] MODULE * should return an unknown command error since not yet implemented
• [x] GETRANGE with non-existing key should return an empty string
• [x] SRANDMEMBER with non-existing key and count specified should return an empty array
• [x] [H|S|Z|CO]SCAN with too few parameters should return an error
• [x] GEO commands (except GEODIST) should return an array of null arrays
• [x] GEOADD with CH and NX options should include added items in output count
• [x] RUNTXP with too few parameters should return an error

Added tests for all cases described

[badrishc]

cc @kevin-montrose