Start work on adding preserve_exact_body_bytes

- Make sure cassettes are 100% compliant
Update options that are validated and their defaults
Fix cassette module and tests for it
Prevent test cassette from being modified every test run
Add backwards compatibility tests
Fix replace_headers method on an Interaction
 Merge branch 'rewrite-pt2' into 'master'

Rewrite Pt 2

Be really certain this time that we're fully compatible with VCR's cassette format.
Fix up headers once more