Fix search widget in SQLFORM.grid for SQLCustomType values

Using a SQLCustomtype in sqlform.grid raises the exception: 
```
Traceback (most recent call last):
  File "/home/pvalleri/src/web2py.ilvalle/gluon/restricted.py", line 224, in restricted
    exec ccode in environment
  File "/home/pvalleri/src/web2py.ilvalle/applications/welcome/controllers/default.py", line 77, in <module>
  File "/home/pvalleri/src/web2py.ilvalle/gluon/globals.py", line 393, in <lambda>
    self._caller = lambda f: f()
  File "/home/pvalleri/src/web2py.ilvalle/applications/welcome/controllers/default.py", line 25, in index
    return dict(grid=SQLFORM.grid(db.product, user_signature=False))
  File "/home/pvalleri/src/web2py.ilvalle/gluon/sqlhtml.py", line 2383, in grid
    search_menu = SQLFORM.search_menu(sfields, prefix=prefix)
  File "/home/pvalleri/src/web2py.ilvalle/gluon/sqlhtml.py", line 1846, in search_menu
    _class=(field.type or '')+' form-control')
TypeError: unsupported operand type(s) for +: 'SQLCustomType' and 'str'
```