Add another strategy for finding Serializers

[tijn]

This strategy looks at the nesting modules of the current namespace. For example:

Api::V3::Frontend::AuthorsController => [
  Api::V3::AuthorSerializer, Api::AuthorSerializer]

As the example shows, this strategy will return ALL the module names that surround the current namespace providing a straightforward way to...

• use different serializers for specialized cases (for when you have similar controllers for the same resource but for different user roles);
• use a specialized serializer in certain controllers and a generic one in all the rest;
• a combination of the above.

Purpose

Allow having multiple specialized serializers while also allowing to reuse generic serializers.

Changes

Added another loopup proc that returns an array of all the namespaces that surround the current one.

Caveats

You have to remember to change the lookup-setting in the config.

Related GitHub issues

Additional helpful information

I'm sorry for not writing any tests for this. I don't really understand the format of the current lookup-chain test. All help is welcome. 😄