Fix dictionary bug when null key has null value

[atifaziz]

This PR fixes a bug introduced by PR #582 in our dictionary implementation that accepts null keys. The bug can be observed via ScanBy:

var nil = (object)null;
var source = new[] { "foo", null, "bar", null, "baz" };
var result = source.ScanBy(c => c, k => nil, (i, k, e) => nil);
foreach (var item in result)
    Console.WriteLine(item);

It prints the following:

[foo, ]
[, ]
[bar, ]

before throwing an ArgumentNullException:

System.ArgumentNullException: Value cannot be null. (Parameter 'key')
   at System.Collections.Generic.Dictionary`2.FindValue(TKey key)
   at System.Collections.Generic.Dictionary`2.TryGetValue(TKey key, TValue& value)
   at MoreLinq.Collections.Dictionary`2.TryGetValue(TKey key, TValue& value) in C:\projects\morelinq\MoreLinq\Collections\Dictionary.cs:line 69
   at MoreLinq.MoreEnumerable.<>c__DisplayClass226_0`3.<<ScanBy>g___|0>d.MoveNext() in C:\projects\morelinq\MoreLinq\ScanBy.cs:line 100

[atifaziz]

@viceroypenguin Thanks for the review!