As an enhancement to #166 , this adds a utility function, feed_options_str, that converts a dict of feed options to the appropriate string value.

This is an optional enhancement: users can supply a string value to feed_options themselves, and this will be passed straight to the request. Otherwise, a dict like the following example:

feed_options = {
    "shippingid": "283845474",
    "totalAmount": 3.25,
    "totalvatamount": 1.23,
    "invoicenumber": "INT-3431-XJE3",
    "documenttype": "CreditNote",
    "transactionid": "amzn:crow:429491192ksjfhe39s",
}

...will be converted to a string by feed_options_str:

print(feed_options_str(feed_options))
>>> "metadata:shippingid=283845474;metadata:totalAmount=3.25;metadata:totalvatamount=1.23;metadata:invoicenumber=INT-3431-XJE3;metadata:documenttype=CreditNote;metadata:transactionid=amzn:crow:429491192ksjfhe39s"

This is in accordance with Amazon documentation, here (PDF) (see 6.4 Invoice Upload Feed Type).

• All keys are prefixed by "metadata:".
• Keys are case-insensitive, so no conversion is performed.
• Values are simply cast to strings, though we make a special case for True and False as in other contexts, converting these literals to "true" and "false", respectively.
• Keys and values are joined as key=value.
• Key-value pairs are joined in the final string with ; as a separator.
• A falsey value for the feed_options dict will return None, instead.
• If the utility is reached with a non-empty non-dict value for feed_options, a ValueError is raised noting the problem.