Add failing test for hasMany relationships

If adapter does not return any data for a hasMany relationship,
the unsaved hasMany records disappear from the relationship.
Fixes #1466
Always keep the hasMany around when payload arrives

We should always keep the current hasMany value if the payload
has no relationships with that key
[Bugfix] Decouple DS.EmbeddedRecordsMixin from DS.ActiveModelSerializer

[Fixes #1974]
Fix class name in debugging message

`RestAdapter` should be `RESTAdapter`.
Merge pull request #2019 from tricknotes/fix-class-name

Fix class name in debugging message
Fixed a loose semicolon

Semicolon was unnecessary and causing a javascript error.
Merge pull request #2022 from rmartinez93/master

Fixed a loose semicolon
updated '_links' to just 'links'
Merge pull request #2024 from dadamssg/master

updated '_links' to just 'links'
fix documentation of `getById`
Merge pull request #2025 from Crunch09/master

fix documentation of `getById`
Merge pull request #1987 from pixelhandler/keyForAttribute-optional

[Bugfix] Decouple EmbeddedRecordsMixin from ActiveModelSerializer
fix documentation of 'attr'
Merge pull request #2027 from JustAboutJeff/DOCS-fix-attr

fix documentation of 'attr'
Added documentation for ajaxError with DS.Errors.
rest_adapter: Remove unused `set` definition
Merge pull request #2033 from duggiefresh/rest_adapter

rest_adapter: Remove unused `set` definition
Stop using deprecated methods

Replace to use some methods that Ember.js recommends.
* `findProperty` -> `findBy`
* `mapProperty` -> `mapBy`
* `filterProperty` -> `filterBy`
* `everyProperty` -> `isEvery`
Merge pull request #2034 from tricknotes/stop-using-deprecated-methods

Stop using deprecated methods