Add support for conditional relationships

[timkomip]

History

Setting the readable option on a relationship to a callables (procs, lambdas, etc) would only evaulate on load (once). Conditionally hiding relationships per request was not supported. Feature was disabled

Current State (before PR)

readable option on relationships (has_many, belongs_to) doesn't support callables (procs, lambdas, etc).

This PR

Adds support for setting readable with callables.

Changes

• Sideload evaluates readable flag every time so that the read conditions can be check against current request (or context)
• Move readable check to serializer. Sets :if option on JSONAPI::Serializable::Relationship object with lambda that calls sideload.readable?
• Apply sideload/relationship without checking readable? on sideload since the check will be happening in the serializer.
• Filters out the include from the include_hash if the current included relationships is not readable.

[jkeen]

@timkomip why are these skipped now?