Remove IdentityMap for good
Remove more WARN from tests
Add docs to Sessions::StorageOptions module
Adding enconding header
Indexes should not be compared only by keys.

Compounded indexes with the same keys, but with different order values,
should be added without condering itself as a duplicate.

[fixes #3348]
Update changelog
Merge pull request #3351 from mongoid/remove_im

Remove IdentityMap for good
Don't require rack/mongoid. It was removed with IdentityMap (7180173).

Fix #3367.
Require ostruct in lib/mongoid/indexable.rb to load OpenStruct.
Merge pull request #3368 from simi/fix-rack

Don't require rack/mongoid. It was removed with IdentityMap (7180173).
Merge pull request #3370 from simi/require-ostruct

Require ostruct in lib/mongoid/indexable.rb to load OpenStruct.
Reduce duplicated MRI Travis builds.
Merge pull request #3371 from simi/reduce-travis

Reduce duplicated MRI Travis builds.
Test ruby-head on Travis CI.
Merge pull request #3373 from simi/test-ruby-head

Test ruby-head on Travis CI.
Simplify Coveralls code
Remove pending test based on identity map.
Merge pull request #3374 from simi/remove-identitymap-pending-test

Remove pending test based on identity map.
Remove some pending tests for record_timestamps.

On mongoid we have Model.timeless, which can make simulate the same
behaviour as record_timestamps on AR, with a bonus of being thread-safe.
Remove multiple jdk tests on CI