fix($filter): $orderBy filter with dates when sorting multiple fields

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How to reproduce: 

Component(s): misc core

Impact: small

Complexity: small

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Request Type: 

How to reproduce: 

Component(s): 

Impact: small

Complexity: small

This issue is related to: 

**Detailed Description:**



**Other Comments:**


extend compare method to use Date.valueOf() when comparing dates

closes #6676