fix: return db name for I_S.TABLES(and others) in original letter case.
if mysql starts with lower_case_table_names=1 | 2 then original db name is converted
to lower case(for I_S tables). It happens when we perform add_table_to_list. 
to avoid this we make a copy of original db name and use the copy hereafter.


mysql-test/r/lowercase_table2.result:
  Bug#17661 information_schema.SCHEMATA returns uppercase with lower_case_table_names = 1
  test case
mysql-test/t/lowercase_table2.test:
  Bug#17661 information_schema.SCHEMATA returns uppercase with lower_case_table_names = 1
  test case