Improve JSON-LD metadata for SEO

[nicolas-bonnel]

Google has launched a Dataset search engine and it relies on JSON-LD metadata to extract informations for their index.

When using their tools, it highlights a lot of errors on pages served by uData, like this one

Looking the Dataset type on schema.org, it seems contributedDistribution should be renamed to distribution for instance.

[noirbizarre]

Right now, we submit extras properties because we haven't found a better solution to expose metadata for properties not yet added to the schema.org/Dataset model.

This is why there is a contributedDistribution in addition to the distribution property to expose "community resources".

The duplicate @type has already been fixed on the current master (will be part of the next 1.6.0 version)

For the extras property, there is an upstream issue open on schema.org to fix that (there is currently no standard way to expose properties unknown to schema.org). See https://github.com/schemaorg/schemaorg/issues/2047