[Core] Don't throw from Response.Content if memory stream is private

Relax our constraint that the underlying buffer of the MemoryStream
backing a buffered response be publicly visible. There are cases today
where `RequestBodyPolicy` will not allocate a new MemoryStream. One
example is if the response stream is already seekable (as is the case
in our PlaybackTransport) nd we've observed that there are likely
cases where `HttpClient` itself may use a MemoryStream as the response
stream without allowing the underling buffer to be exposed.

In cases where we can not extract the underlying body, just make a
copy of it. Since we know the underlying stream is a memory stream, we
don't need to worry about hidden IO.

Fixes #21048

sdk/core/Azure.Core/CHANGELOG.md

@@ -6,6 +6,10 @@
 
 - Types to represent `GeoJson` primitives.
 
+### Changed
+
+- `Response.Content` no longer throws `InvalidOperationException` when the response is backed by a `MemoryStream` with a non publicly visible buffer.
+
 ## 1.14.0 (2021-05-11)
 
 ### Features Added

sdk/core/Azure.Core/src/Response.cs

@@ -72,7 +72,7 @@ public virtual BinaryData Content
                 }
                 else
                 {
-                    throw new InvalidOperationException($"The {nameof(ContentStream)}'s internal buffer cannot be accessed.");
+                    return new BinaryData(memoryContent.ToArray());
                 }
             }
         }

sdk/core/Azure.Core/tests/ResponseTests.cs

@@ -123,12 +123,15 @@ public void ContentPropertyThrowsForNonMemoryStream()
             Assert.Throws<InvalidOperationException>(() => { BinaryData d = response.Content; });
         }
 
-        public void ContentPropertyThrowsForNotExposableMemoryStream()
+        [Test]
+        public void ContentPropertyWorksForMemoryStreamsWithPrivateBuffers()
         {
             var response = new MockResponse(200);
-            response.ContentStream = new MemoryStream(new byte[100], 0, 100, writable: false, publiclyVisible: false);
+            var responseBody = new byte[100];
+            response.ContentStream = new MemoryStream(responseBody, 0, responseBody.Length, writable: false, publiclyVisible: false);
 
-            Assert.Throws<InvalidOperationException>(() => { BinaryData d = response.Content; });
+            Assert.DoesNotThrow(() => { BinaryData d = response.Content; });
+            CollectionAssert.AreEqual(responseBody, response.Content.ToArray());
         }
 
         internal class TestPayload