[FSSDK-9472] fix: Last-Modified header not respected (#355)

* Fix where Last-Modified is pulled

* Corrected small problems in test class

* WIP Adding Last Modified test

* Add test coverage

WIP: new tests succeed in isolation (time-based/brittle)

* Fix failing test by deferring

* Lint fixes

* Lint fix whitespace

Author: mikechu-optimizely