fix broken explain on oracle due exec_xxx refactoring

Author: kares

lib/arjdbc/oracle/adapter.rb

@@ -535,7 +535,7 @@ def supports_explain? # :nodoc:
     def explain(arel, binds = [])
       sql = "EXPLAIN PLAN FOR #{to_sql(arel)}"
       return if sql =~ /FROM all_/
-      exec_query(sql, 'EXPLAIN', binds)
+      execute(sql, 'EXPLAIN', binds)
       select_values("SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY)", 'EXPLAIN').join("\n")
     end