for same data when using bit fields

Problem: checksum for BIT fields may be computed incorrectly 
in some cases due to its storage peculiarity.

Fix: convert a BIT field to a string then calculate its checksum.


mysql-test/r/myisam.result:
  Fix for bug#51304: checksum table gives different results 
  for same data when using bit fields
    - test result.
mysql-test/t/myisam.test:
  Fix for bug#51304: checksum table gives different results 
  for same data when using bit fields
    - test case.
sql/sql_table.cc:
  Fix for bug#51304: checksum table gives different results 
  for same data when using bit fields
    - convert BIT fields to strings calculating its checksums
  as some bits may be saved among NULL bits in the record buffer.