[BUGFIX] fixture adapter copies defined fixtures

Otherwise, they will get mutated by Ember Data
in various serialize hooks.

fixes GH-1454
Add support for embedded polymorphic hasMany
Merge pull request #2217 from emberjs/1454-fixture-adapter-deep-copy

[BUGFIX] fixture adapter copies defined fixtures
Merge pull request #2152 from hooktstudios/embeddedHasManyPolymorphic

Add support for embedded polymorphic hasMany
Add ajaxSuccess hook to the RESTAdapter

Previously there was no easy way to extract data from response headers
or to reject promises after a 200OK response.
temp
tests and typo fix for ajaxSuccess/ajaxError
Merge pull request #1968 from emberjs/ajaxSuccess

Add ajaxSuccess hook to the RESTAdapter