become unescaped

Problem was only with the print out of the KEY partitioning list of
fields, it did not include quotes, even if it was needed.

Fixed by always add quotes if needed.

mysql-test/r/partition.result:
  Bug#39338: Fieldnames in INFORMATIONSCHEMA.PARTITIONS.PARTITION_EXPRESSION
  become unescaped
  
  Updated result
mysql-test/t/partition.test:
  Bug#39338: Fieldnames in INFORMATIONSCHEMA.PARTITIONS.PARTITION_EXPRESSION
  become unescaped
  
  Added test case
sql/sql_show.cc:
  Bug#39338: Fieldnames in INFORMATIONSCHEMA.PARTITIONS.PARTITION_EXPRESSION
  become unescaped
  
  Added quotes if necessary.