fix RSS feed aggregation broken by SimplePie 1.9 upgrade

SimplePie 1.9 added a FileClient that rejects any response whose error is
non-null while the status code is zero. FeedParserFile never set a

fix RSS feed aggregation broken by SimplePie 1.9 upgrade

SimplePie 1.9 added a FileClient that rejects any response whose error is
non-null while the status code is zero. FeedParserFile never set a status
code and copied DokuHTTPClient's error verbatim, which is an empty string
(not null) on success - so every successful fetch was misclassified as a
failed request and feeds rendered the 'rssfailed' message.

Populate the response through SimplePie's Response interface methods backed
by our own properties (status code, body, normalized headers, requested URL)
and report a missing error as null, instead of writing File's deprecated
backwards-compatibility properties.

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