Rank has incorrect output for equivalent values

[viceroypenguin]

Given the following sequence: new[] { 2, 2, 1, 1, 3, 3, }

Currently Rank produces the following sequence: { 3, 4, 1, 2, 5, 6, }.

However, equivalent values should have equivalent ranks. Any desire to have differentiate between values should be done as part of the input sequence and/or the comparer.

The proper result for Rank should be: { 3, 3, 1, 1, 5, 5, }. If gaps are not desired, then DenseRank may be added which would return: { 2, 2, 1, 1, 3, 3, }.

CC: @AmbientLion

[viceroypenguin]

Duplicate of #826