Try to retain existing cartodb_id values

Also reset sequence when original cartodb_id values correctly
cast to integers, and does not keep a backup in that case.

Closes #27

test/CDB_CartodbfyTableTest.sql

@@ -178,6 +178,18 @@ SELECT CDB_CartodbfyTableCheck('t', 'text timestamps');
 SELECT reftime-created_at, reftime-updated_at FROM t;
 DROP TABLE t;
 
+-- table with existing cartodb_id field ot type text
+CREATE TABLE t AS SELECT 10::text as cartodb_id;
+SELECT CDB_CartodbfyTableCheck('t', 'text cartodb_id');
+select cartodb_id/2 FROM t; 
+DROP TABLE t;
+
+-- table with existing cartodb_id field ot type text not casting
+CREATE TABLE t AS SELECT 'nan' as cartodb_id;
+SELECT CDB_CartodbfyTableCheck('t', 'uncasting text cartodb_id');
+select cartodb_id,_cartodb_id0 FROM t; 
+DROP TABLE t;
+
 -- TODO: table with existing custom-triggered the_geom
 
 DROP FUNCTION CDB_CartodbfyTableCheck(regclass, text);

test/CDB_CartodbfyTableTest_expect

@@ -29,5 +29,13 @@ SELECT 1
 text timestamps cartodbfied fine
 @ 0|@ 0
 DROP TABLE
+SELECT 1
+text cartodb_id cartodbfied fine
+5
+DROP TABLE
+SELECT 1
+uncasting text cartodb_id cartodbfied fine
+1|nan
+DROP TABLE
 DROP FUNCTION
 DROP FUNCTION