Fix handling of _raw field in ResultsReaders.

_raw is returned as plain text when accessed as a field on an event, and events have a new method
raw_as_xml which returns the full XML of _raw, including highlighting.
Make export tests use an ExportStream wrapped input, not just a raw file.

We weren't testing what we thought we were testing, since there is now special handling
for export streams in ResultsReader.
Resolved code review comments.
Merge branch 'fross/xml_work' into develop