"_links" JSON field serialization convention fix

Special case JSON _links property.
    
    For some reason, the List Feeds http://developer.github.com/v3/activity/feeds/#list-feeds
    API response has a field that starts with an underscore. This also occurs in
    the Review Comments API: http://developer.github.com/v3/pulls/comments/#list-comments-in-a-repository
    and in the Contents API. http://developer.github.com/v3/repos/contents/#get-the-readme
    
    This commit special cases that one field so we can use the expected CLR
    property name. I couldn't find a case where a
    JSON response doesn't prefix "links" with an underscore.
    
    Related to #386