Add option to configure FaradayMiddleware::Mashify mash_class

[mattruzicka]

I needed something like this in order to turn off this warning in the logs:

WARN -- : You are setting a key that conflicts with a built-in method Hashie::Mash#count defined in Enumerable. This can cause unexpected behavior when accessing the key via as a property. You can still access the key via the #[] method.

I put this code in an initializer:

class InstaMash < ::Hashie::Mash
  disable_warnings
end

Instagram.configure do |config|
  config.mash_class = InstaMash
end

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[icco]

Checkout https://github.com/icco/instagram-continued, this may have already fixed your issue.

[zhu1230]

see icco#17, this PR fixed the issue.