Menu value set with item value

[radagi]

In case items have 'value' attributes they are used to set the menu 'value'. Doing so also the dropdown menus differenziate between menu label and menu value.

With this fix 'Cars' fields in the iron-form demo for both 'GET' 'and 'POST' cases can generate the same submit parameter (After having associated the right 'value' attributes to the paper-items).

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[radagi]

I signed the CLA.

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[cdata]

Hi, thank you for your contribution. I'm closing this because it's a dupe of #104 and related PR. Also, it does not follow our guidelines for contributing PRs. Please consult our contribution guidelines for help on filing issues and opening PRs in the future!