truncates text/blob to 766 chars

mysqldump and SELECT ... INTO OUTFILE truncated long BLOB/TEXT
values to size of 766 bytes (MAX_FIELD_WIDTH or 255 * 3 + 1).

The select_export::send_data method has been modified to
reallocate a conversion buffer for long field data.


mysql-test/r/mysqldump.result:
  Test case for bug #53088.
mysql-test/r/outfile_loaddata.result:
  Test case for bug #53088.
mysql-test/t/mysqldump.test:
  Test case for bug #53088.
mysql-test/t/outfile_loaddata.test:
  Test case for bug #53088.
sql/sql_class.cc:
  Bug #53088: mysqldump with -T & --default-character-set set
              truncates text/blob to 766 chars
  
  The select_export::send_data method has been modified to
  reallocate a conversion buffer for long field data.