Merge pull request #2330 from Clockwork-Muse/PLINQ_Zip

PLINQ - Correct type used in constructor.

Author: stephentoub

src/System.Linq.Parallel/src/System/Linq/Parallel/QueryOperators/Binary/ZipQueryOperator.cs

@@ -43,7 +43,7 @@ internal sealed class ZipQueryOperator<TLeftInput, TRightInput, TOutput>
         //
 
         internal ZipQueryOperator(
-            ParallelQuery<TLeftInput> leftChildSource, IEnumerable<TRightInput> rightChildSource,
+            ParallelQuery<TLeftInput> leftChildSource, ParallelQuery<TRightInput> rightChildSource,
             Func<TLeftInput, TRightInput, TOutput> resultSelector)
             : this(
                 QueryOperator<TLeftInput>.AsQueryOperator(leftChildSource),