Added a test case for bug #7098.
sql_select.cc:
  Fixed bug #7098.
  When a string field was substituted for an equal constant
  the collation of the constant was changed by mistake for 
  the binary collation.


sql/sql_select.cc:
  Fixed bug #7098.
  When a string field was substituted for an equal constant
  the collation of the constant was changed by mistake for 
  the binary collation.
mysql-test/t/select.test:
  Added a test case for bug #7098.
mysql-test/r/select.result:
  Added a test case for bug #7098.