MaxByOrDefault

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Currently, MaxBy will throw an exception if executed on an empty enumerable.  I 
would like a way to provide it with a default value to use, or have it use 
default(TSource).

Example usage:
var emptyEnumerable = Enumerable.Empty<Person>();
var result = emptyEnumerable.MaxByOrDefault(person => person.Age);
Assert.Equal(0, result);

result = emptyEnumerable.MaxBy(person => person.Age, 10);
Assert.Equal(10, result);

Original issue reported on code.google.com by mi...@zoltu.net on 8 Dec 2014 at 8:12