Added assertion for updateLink

Added an assertion for string type. Since you could also pass objects
to `updateLink` (which would always result in false in the next if
statement) containing a link. This PR notifies creators of serializers
so they always use a string when handling links.

Added test for links with type other than string
Removed a duplicate line.
Added test case for push link with null value
[DOC] Update usage of AMS

`include` option is renamed as `embed_in_root`
in current stable version 0.9.
* https://github.com/rails-api/active_model_serializers/blob/0-9-stable/lib/active_model/serializer/association.rb#L11
Merge pull request #2383 from tricknotes/update-usage-of-ams

[DOC] Update usage of AMS
Remove unnecessary `Function#call`
[DOC] Fix to apply js doc style to DS.ManyArray
[DOC] Fix usage of DS.Model.fields

`DS.Model.fields` is a `Ember.Map`.
Its behavior changed at 2eb1c971 .
Merge pull request #2388 from tricknotes/doc

[DOC] Fix to apply js doc style
Merge pull request #2387 from tricknotes/remove-unnecessary-call

Remove unnecessary `Function#call`
Make dependencies clearer.

This is mostly just for easier discovery/understanding for passers by
looking at the `bower.json` in the root of the repo. The actual
`bower.json` that we put in components/ember-data is correct already.
Merge pull request #2390 from rwjblue/deps

Make dependencies clearer.
Refactored InvalidError handling into a serializer concern.
Merge pull request #2392 from ahacking/serialization-for-invalid-error

Refactored InvalidError handling into a serializer concern.
Merge pull request #2354 from steffenbrem/update-links-assert-fix

Added assertion for updateLink
Fixing an issue when grouping requests could result in URL longer than configured length.
Merge pull request #2396 from grishace/uri_encoded_brackets

Fixing an issue when grouping requests could result in long URL
Nicer errors when pushing belongsTo/hasMany with invalid values
Merge pull request #2377 from wecc/nicer-push-errors

Nicer errors when pushing belongsTo/hasMany with invalid values
Directly reloading a hasMany with links should trigger only one request

Introducing a loading promise in the many relationship
in order to keep track of its loading state.