Fix partial requests (used by fetch -r) when the requested file is
already complete.

Since 416 is an error code, any Content-Range header in the response
would refer to the error message, not the requested document, so
relying on the value of size when we know we got a 416 is wrong.
Instead, just verify that offset == 0 and assume that we've reached
the end of the document (if offset > 0, we did not request a range,
and the server is screwing with us). Note that we cannot distinguish
between reaching the end and going past it, but that is a flaw in the
protocol, not in the code, so we just have to assume that the caller
knows what it's doing. A smart caller would request an offset
slightly before what it believes is the end and compare the result to
what is already in the file.

PR: 212065
Reported by: mandree
MFC after: 3 weeks