Fix the cache key for complex querystrings

Issue:

When the querystring includes params that are actually arrays (i.e. ?_where[0]=...&where[1]=... (See: https://strapi.io/documentation/v3.x/content-api/parameters.html#complex-queries), the current implementation just calls toString on the value (by way of template literals), which results in [object Object] in the cache key and hence breaking the cache.

Thanks to @bartzy for the fix (#28)