Fixed GetEnumerator() method on mock (dotnet#3999)

Author: mjeldreth

entity-framework/ef6/fundamentals/testing/mocking.md

@@ -207,7 +207,7 @@ namespace TestingDemo
             mockSet.As<IQueryable<Blog>>().Setup(m => m.Provider).Returns(data.Provider);
             mockSet.As<IQueryable<Blog>>().Setup(m => m.Expression).Returns(data.Expression);
             mockSet.As<IQueryable<Blog>>().Setup(m => m.ElementType).Returns(data.ElementType);
-            mockSet.As<IQueryable<Blog>>().Setup(m => m.GetEnumerator()).Returns(data.GetEnumerator());
+            mockSet.As<IQueryable<Blog>>().Setup(m => m.GetEnumerator()).Returns(() => data.GetEnumerator());
 
             var mockContext = new Mock<BloggingContext>();
             mockContext.Setup(c => c.Blogs).Returns(mockSet.Object);
@@ -382,7 +382,7 @@ namespace TestingDemo
 
             mockSet.As<IQueryable<Blog>>().Setup(m => m.Expression).Returns(data.Expression);
             mockSet.As<IQueryable<Blog>>().Setup(m => m.ElementType).Returns(data.ElementType);
-            mockSet.As<IQueryable<Blog>>().Setup(m => m.GetEnumerator()).Returns(data.GetEnumerator());
+            mockSet.As<IQueryable<Blog>>().Setup(m => m.GetEnumerator()).Returns(() => data.GetEnumerator());
 
             var mockContext = new Mock<BloggingContext>();
             mockContext.Setup(c => c.Blogs).Returns(mockSet.Object);