DecimalWidget should be initialized from text

[andreynovikov]

As per doc:

>>> Decimal('3.14')
Decimal('3.14')
>>> Decimal(3.14)
Decimal('3.140000000000000124344978758017532527446746826171875')

When I've changed this line:

django-import-export/import_export/widgets.py

Line 88 in 3cf5e3f

return Decimal(value)

to

        return Decimal(force_text(value))

the import of value 1.4 changed from:

1.40399999999999999911182158029987476766109466552734375

to:

1.40