Update Simple.Data/QueryPolyfills/DictionaryQueryRunner.cs

Running order by clauses in reverse order to fix issue with Order By/Then by for in-memory adapter. #252:
https://github.com/markrendle/Simple.Data/issues/252
Update Simple.Data.InMemoryTest/InMemoryTests.cs

Added order by/then by test
Merge pull request #260 from mattwolin/master

Fix OrderBy/Then By issue with In-Memory adapter. Issue #252