- if there are two character set definitions in the column declaration,
    we replace the first one with the second one as we store both in the LEX->charset
    slot. Add a separate slot to the LEX structure to store underscore charset.
  - convert default values to the column charset of STRING, VARSTRING fields 
    if necessary as well.


mysql-test/r/ctype_recoding.result:
  Fix for bug #20695: Charset introducer overrides charset definition for column.
    - test result.
mysql-test/t/ctype_recoding.test:
  Fix for bug #20695: Charset introducer overrides charset definition for column.
    - test case.
sql/sql_lex.cc:
  Fix for bug #20695: Charset introducer overrides charset definition for column.
    - LEX->underscore_charset introduced to store UNDERSCORE_CHARSET
sql/sql_lex.h:
  Fix for bug #20695: Charset introducer overrides charset definition for column.
    - LEX->underscore_charset introduced to store UNDERSCORE_CHARSET
sql/sql_table.cc:
  Fix for bug #20695: Charset introducer overrides charset definition for column.
    - convert default values to the column charset of VARSTRING, STRING, ENUM, 
      SET fields if necessary.
sql/sql_yacc.yy:
  Fix for bug #20695: Charset introducer overrides charset definition for column.
    - LEX->underscore_charset introduced to store UNDERSCORE_CHARSET