Fix ISO formatting for modified datetime.

This change will make sure that format_modified returns datetime value
in ISO 8601 format. See W3C documentation at
http://www.w3.org/TR/NOTE-datetime.

Since ```modified``` is parsed and then converted to UTC after it's
extracted from HTTP response, it's safe to assume that format_modified
will always format UTC datetime values.

Instead of ```isoformat``` method which omits timezone information for
UTC values, ```strftime``` with a specific format (that ends with Z)
will be used.
Merge pull request #426 from alperkokmen/fix-lastmod-format

Fix ISO formatting for modified datetime.