Contextual memory multi field sort

I believe the spec for the multi field sort in ```Mongoid::Contextual::Memory``` was wrong.

Looking at the documentation it seems calling ```context.sort(number: 1, street: 1)``` should sort by number first and only then by street, this should result in the following order: 

Street       |  Number
-----------   | ---------
friedel      |   1
pfluger     |   5
hobrecht |   9

while the spec before this fix expects:

Street       |  Number
-----------   | ---------
friedel      |   1
hobrecht |   9
pfluger     |   5


I've also taken the time to add Lenaustraße to the test with a conflicting house number to make sure that the second sorting really works. the expected result is then:

Street       |  Number
-----------   | ---------
friedel      |   1
lenau       |   5
pfluger     |   5
hobrecht |   9

Let me know if this requires any other fix or that I was wrong to begin with about the way it should work.

Thanks!