get_statistics_subset should return well-formatted null results

The existing code for handling lists of values that don't meet minimum
length causes problems when the `percentiles` key is used; e.g., rather
than the expected

    [{percentile, [{0.5, 0.0}, ... ]}]

the user is presented with

    [{{percentile, [0.5, ... ]}, 0.0}]

which doesn't match the formatting for other subset key. This patch
special-cases the `percentile` key to return the expected result.

src/bear.erl

@@ -114,7 +114,14 @@ get_statistics_subset([_,_,_,_,_|_] = Values, Items) ->
 			  SortedValues, Scan_res, Scan_res2)
     end;
 get_statistics_subset(Values, Items) when is_list(Values) ->
-    [{Item, 0.0} || Item <- Items].
+    get_null_statistics_subset(Items, []).
+
+get_null_statistics_subset([{percentile, Ps}|Items], Acc) ->
+    get_null_statistics_subset(Items, [{percentile, [{P, 0.0} || P <- Ps]}|Acc]);
+get_null_statistics_subset([I|Items], Acc) ->
+    get_null_statistics_subset(Items, [{I, 0.0}|Acc]);
+get_null_statistics_subset([], Acc) ->
+    lists:reverse(Acc).
 
 calc_steps(Items) ->
     lists:foldl(fun({I,_},Acc) ->