Wrong right justified receiving_dfi_account_number field

[fangpenlin]

The field

https://github.com/bninja/nacha/blob/master/nacha/__init__.py#L265

should be left justified by NACHA documents, that's why Wells Fargo said that they saw some strange space padded in front account number.

However, I bet the implementation would be something like account_number.strip(), so it happens to work anyway at least for Wells Fargo.