Added test case for bug #11283.
field.h, field.cc:
  Fixed bug #11283: wrong conversion from varchar to decimal.
  Added methods Field_string::val_decimal,
  Field_varstring::val_decimal, Field_blob::val_decimal.
  They are not inherited from the base class Field_longstr
  anymore.


sql/field.cc:
  Fixed bug #11283: wrong conversion from varchar to decimal.
  Added methods Field_string::val_decimal,
  Field_varstring::val_decimal, Field_blob::val_decimal.
  They are not inherited from the base class Field_longstr
  anymore.
sql/field.h:
  Fixed bug #11283: wrong conversion from varchar to decimal.
  Added methods Field_string::val_decimal,
  Field_varstring::val_decimal, Field_blob::val_decimal.
  They are not inherited from the base class Field_longstr
  anymore.
mysql-test/t/cast.test:
  Added test case for bug #11283.
mysql-test/r/cast.result:
  Added test case for bug #11283.