[Union] fix Reset for UnionEnumerator and RefUnionEnumerator.

Author: reegeek

src/StructLinq/Union/RefUnionEnumerator.cs

@@ -60,6 +60,7 @@ public void Reset()
             set.Clear();
             enumerator1.Reset();
             enumerator2.Reset();
+            first = true;
         }
 
         public ref T Current

src/StructLinq/Union/UnionEnumerator.cs

@@ -61,6 +61,7 @@ public void Reset()
             set.Clear();
             enumerator1.Reset();
             enumerator2.Reset();
+            first = true;
         }
 
         public T Current