Fix assymetry between converting a PDU to bytes and converting bytes to a PDU.

[exhuma]

When using pop_tlv to convert a bytes object to a bytes object into a PDU object, the result is different than what you might expect.

• Instantiating a PDU class takes pure Python integer objects as initialisor arguments.
• When converting a PDU using pop_tlv these values are represented as puresnmp Integer instance.

This has been observed with the error_index and error_status fields.