Field_string::val_int(), Field_string::val_real(), Field_string::val_decimal()
passed the whole buffer of field_length bytes to data type conversion routines.
This made conversion routines to print redundant trailing spaces in case of warnings.

Adding a method Field_string::to_lex_cstring() and using it inside
val_int(), val_real(), val_decimal(), val_str().

After this change conversion routines get the same value with what val_str() returns,
and no redundant trailing spaces are displayed.