* revisit loading fix for revisit records with http headers:
- if revisit record has http headers, always use those headers
- otherwise, continue to use http headers from payload record
- parse headers of http and payload records on initial lookup, to simplify loading
- tests: add test for loading revisit records with different urls, different headers but same payload
- fix for sul-dlss/was-pywb#64
* also bump version to 2.6.8
