Fix ModifiedAfter filter format

[anukul]

From Xero API docs:

The ModifiedAfter filter is actually an HTTP header: ' If-Modified-Since'.
A UTC timestamp (yyyy-mm-ddThh:mm:ss)

PyXero does not follow this format for the timestamp, as seen here.

[Zylatis]

Bumping this one up, mostly because I'd like to understand how to pass this constraint to the contacts.filter() method so as to only get contacts where the balance owed has changed in the last month

[schinckel]

Can you supply a link for the documentation? The HTTP specification asserts that this header should not be an ISO8601 timestamp.

(I'll take it up with Xero developer relations if I can find a link documenting it).