Add support for giving back total count via header

[mkg20001]

This adds a new includeTotalCount flag that by default exposes the current range and total count as specified in content-range and a property totalCountHeader, which if set to anything other than 'content-range' will add a header that contains just the total number of elements

This is more or less an attempt to be compatible with whatever https://www.npmjs.com/package/ra-data-simple-rest expects as return value

Note: this is incomplete as it's missing support in other tandys with count support

[mattboutet]

If you'd be willing to flesh this out further to hit the other spots with count support and add documentation to the README, I'd be happy to merge this.

[coveralls]

Pull Request Test Coverage Report for Build 102

• 51 of 55 (92.73%) changed or added relevant lines in 10 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.3%) to 98.611%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/actions/populate.js	13	17	76.47%

Totals
Change from base Build 99:	-1.3%
Covered Lines:	284
Relevant Lines:	288

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 101

• 30 of 30 (100.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 99.861%

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Totals
Change from base Build 99:	-0.03%
Covered Lines:	718
Relevant Lines:	719

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💛 - Coveralls

[mkg20001]

hrm... in populate modelFull[options.associationAttr] is used to get the length but later modelFull is returned. Does it make sense to return the full object? I mean only the association (the array of items) is/should be populated.

That way it's easier to work with the data, since just the array is returned. Esp since we also do pagination, that seems odd.

Then it might make more sense to rewrite the query so it just uses the base object for the join but doesn't select anything from it.

[mkg20001]

I have some ideas on populate: Currently it's only possible to populate one specific key. The user may want to populate some or all of them. And for a particular object. So it may make sense to provide a query parameter for find-by-id (or even find aswell) where the user can specify "*" or a comma-seperated list of association attributes to populate.