RFR 7128648: HttpURLConnection.getHeaderFields should return an unmodifiable Map

[Chris Hegarty]

Since the integration of the HTTPOnly changes, CR 7095980, the map 
returned by HttpURLConnection.getHeaderFields is not unmodifiable.

This is contradaction to the API specification. 
URLConnection.getHeaderFields() : "Returns an unmodifiable Map of the 
header fields. The Map keys are Strings that represent the 
response-header field names. Each Map value is an unmodifiable List of 
Strings that represents the corresponding field values."

http://cr.openjdk.java.net/~chegar/7128648/webrev.00/webrev/

Thanks,
-Chris.