wrong result for DML
When making key reference buffers over CHAR fields whitespace (0x20) 
must be used to fill in the remaining space in the field's buffer.
This is what Field_string::store() does.
Fixed Field_string::get_key_image() to do the same.


mysql-test/r/innodb_mysql.result:
  Bug#28878: test case
mysql-test/t/innodb_mysql.test:
  Bug#28878: test case
sql/field.cc:
  Bug#28878: Fill with space instead of binary zeros.