when loading a PDF as a page, the browser returns a 'false positive'
net::ERR_ABORTED even though the PDF is loaded.
- this is already handled, but status code was still being cleared,
ensure status code is not reset to 0 on response
- ensure page status and mime are also recorded if this failure is
ignored (in shouldIgnoreAbort)
- tests: add test for PDF capture
fixes#570
