Fixes with Fred's test changes

1. Implemented a new public method to return XML fragment of "_raw"
field, which may have segmentation markers i.e. <sg> elements. Added
test code for it.
2. Changed test expected files for xml escaping and validation for the
method mentioned above.
3. Fixed a code regression from earlier code cleanup.
Note that one ATOM test (part of Fred's test suite) fails. It is for
Splunk version prior to 4.3., where 'Feed' element is missing from
Splunk server. We don't handle this in the SDK. I'll open a JIRA item
for tracking.