Fixes hasMany records being duplicated after payload returns from server

There is an edge case where a hasMany record gets duplicated (same ID).
Basically, if we generate a record locally using UUID or any custom ID
format, then push it to production which will return the same ID
previously defined, the store's array that holds records to be resolved
is going to duplicate both items, even if they have the same ID.

This commit fixes that by not allowing duplicated IDs in that array.
documentation fix

should be string 'no'
Merge pull request #1998 from kingpin2k/master

Documentation fix for embedded records
Merge pull request #1976 from emberjs/pr1656

Fixes hasMany records being duplicated after payload returns from server
Add example for expected JSON relationship keys.
Fixes typo in `pushPayload` documentation
Merge pull request #2001 from locks/patch-2

Fixes typo in `pushPayload` documentation
Merge pull request #2000 from efigarolam/master

Add example for expected JSON relationship keys.
[Bugfix] Decouple DS.EmbeddedRecordsMixin from DS.ActiveModelSerializer

[Fixes #1974]