ONLY_FULL_GROUP_BY

Problem was that during checking and preparation of the
partitioining function as a side effect in fix_fields
the full_group_by_flag was changed.

Solution was to set it back to its original value after
calling fix_fields.

Updated patch, to also exclude allow_sum_func from being
affected of fix_fields, as requested by reviewer.

mysql-test/r/partition.result:
  Bug#46923: select count(*) from partitioned table fails with
  ONLY_FULL_GROUP_BY
  
  Updated result file
mysql-test/t/partition.test:
  Bug#46923: select count(*) from partitioned table fails with
  ONLY_FULL_GROUP_BY
  
  Extended test case to cover this bug
sql/sql_partition.cc:
  Bug#46923: select count(*) from partitioned table fails with
  ONLY_FULL_GROUP_BY
  
  Resetting full_group_by_flag and allow_sum_func
  back to their original values,
  not conflicting with the sql_mode 'ONLY_FULL_GROUP_BY'