Use lowercased comparison for benchmark fields

For case insensitive comparison we were lowercasing the fields in the
benchmark csv header but not the fields being compared. So the fields
were not matching if we passed them in mixed case.

benchmark/Chart.hs

@@ -186,7 +186,7 @@ main = do
                     { presentation = Groups PercentDiff
                     , selectBenchmarks = selectBench (sortByName opts)
                     , selectFields = filter
-                        ( flip elem fs
+                        ( flip elem (fmap (fmap toLower) fs)
                         . fmap toLower
                         )
                     }