"Invalid Input" error on VendorOrders get_purchase_orders() depending on changedAfter value #433

Describe the bug
Calls to VendorOrders().get_purchase_orders() succeed or fail depending on the value of changedAfter.

To Reproduce
This call succeeds:

VendorOrders().get_purchase_orders(changedAfter="2022-03-05T15:15:48.001000+00:00", nextToken=None)

This call fails:

VendorOrders().get_purchase_orders(changedAfter="2022-03-04T21:00:56.001000+00:00", nextToken=None)

The error response is:

SellingApiBadRequestException: [{'code': 'InvalidInput', 'message': 'Invalid Input', 'details': ''}]

Notice that the only difference is the changedAfter value - both are in ISO-8601 date/time format and both are well within the 7 day range stated in the documentation. These calls also fail:

VendorOrders().get_purchase_orders(changedAfter="2022-03-04T21:00:56.001000", nextToken=None)
VendorOrders().get_purchase_orders(changedAfter="2022-03-04T21:00:56.001", nextToken=None)
VendorOrders().get_purchase_orders(changedAfter="2022-03-04T21:00:56", nextToken=None)

Expected behavior
It's expected that these calls should succeed as long as the changedAfter date is ISO-8601 date/time format and within the 7 day range.

Additional context
I suspect these issues are with the Amazon REST API and not this SDK, but it's hard to know for sure. Also, the API error response message is useless. I also suspect this is the fault of the Amazon API and not this SDK but again, it's hard to know for sure.