Fix for x-ms-date format issue with SDF in GatewayProxy.java

The simple date format used in computing the x-ms-date header uses Week year instead of just year. 
See this http://stackoverflow.com/questions/8686331/y-returns-2012-while-y-returns-2011-in-simpledateformat
We got the SDK working till last week, but today it was not, the reason is, the year from 'YYY' is coming as DEC 2015, but supposed to be DEC 2014. These failures would happen during last week of the year or first week. 

--Exception from SDK --
WARNING: Operation will NOT be retried. Exception: The input date header is invalid format. Please pass in RFC 1123 style date format.
com.microsoft.azure.documentdb.DocumentClientException: The input date header is invalid format. Please pass in RFC 1123 style date format.
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I made a small change to the simple date format from 'YYY' to 'yyyy', please review.