Fix empty value attribute

[ericvaladas]

Something changed in React in version 16.3 that caused the value attribute to appear on an input when it had a defaultValue of undefined or null. In previous versions the attribute would be omitted unless it had a value.

For example, this JSX:

<input name="test" defaultValue={undefined}/>

will generate this HTML:

<input name="test" value>

whereas in versions prior to 16.3, it would generate this HTML:

<input name="test">

This change can cause an issue with checkbox values. When the value attribute is omitted on a checkbox, the target value will be "on" when checked. When a checkbox has the value attribute set, the target value will be that of the attribute when checked. In the case where the attribute is present without a value, the target value will be "" when checked, which is a falsey value.

This PR implements some logic for setting the value and defaultValue on child elements only when needed to workaround this issue. Also, checkboxes will now have a value of true when checked and the value attribute is empty.

[coveralls]

Pull Request Test Coverage Report for Build 283

• 10 of 10 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 279:	0.0%
Covered Lines:	123
Relevant Lines:	123

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💛 - Coveralls