Fixed fragment_cached? method to check if caching

[bdmac]

I noticed that fragment caching does not actually check if caching is
enabled as it seemingly should.

The way CachedSerializer#fragment_cached? worked previously would return
true even in an environment where caching was disabled as defined by
ActiveModelSerializers.config.perform_caching.

Added check for _cache like in the cached? method before checking
whether _cache_only or _cache_except is set.

There were no existing tests for any of these methods but it's a pretty
trivial change.

[bdmac]

BTW this happens because the implementation of the cache DSL sets the various options (in this case specifically _cache_only and _cache_except regardless of whether or not caching is actually enabled. Relevant code for reference:

        def cache(options = {})
          self._cache = ActiveModelSerializers.config.cache_store if ActiveModelSerializers.config.perform_caching
          self._cache_key = options.delete(:key)
          self._cache_only = options.delete(:only)
          self._cache_except = options.delete(:except)
          self._cache_options = (options.empty?) ? nil : options
        end

[bf4]

I guess the idea was to check if fragment_caching was available independent of whether or not caching is available

[bf4]

really, the block in cache_check should check for @klass._cache first, I think. return yield unless @klass._cache guard clause or something

[bdmac]

Right maybe except I don't think you can fragment cache if caching isn't available so that seems moot. I think the answer to that question shouldn't be relevant if caching is disabled.

[bf4]

@bdmac Would you mind adding a test for this and a changelog entry at the top of 'fixes'?

[bf4]

thanks #1477 (comment)

[bf4]

Merged with changelog and tests added in 2789cc5