serializeForm duplicates values when element inside iron-selector

[LordOfTheRams]

Description

When using iron-forms and putting paper-checkboxes inside a iron-selector and then serializing the form the selected values get duplicated. Without the iron-selector it works fine. I think this is a serialzation issue.

<iron-selector attr-for-selected="value" selected-attribute="checked"  multi>
   <paper-checkbox name="category" value="Arts" >Arts</paper-checkbox>
   <paper-checkbox name="category" value="Design">Design</paper-checkbox>
</iron-selector>

Expected outcome

When selecting 'Arts':

{"category":["Arts"]}

Actual outcome

{"category":["Arts","Arts"]}

Steps to reproduce

Put paper-checkbox inside iron-selector and call serializeForm()

Browsers Affected

• Chrome
• Firefox
• Safari 9
• Safari 8
• Safari 7
• Edge
• IE 11
• IE 10

[ianjosephwilson]

I think this is the same bug as #216