Merge pull request dotnet/corefx#9119 from stephentoub/fix_httpstress

Fix HttpClientMiniStressTests tests

Commit migrated from https://github.com/dotnet/corefx/commit/8bba8b289e8f3c37931c999ba83da8f5d8d9efe1

src/libraries/System.Net.Http/tests/FunctionalTests/HttpClientMiniStressTest.cs

@@ -213,9 +213,9 @@ public async Task UnreadResponseMessage_Collectible()
             $"HTTP/1.1 200 OK\r\n" +
             $"Date: {DateTimeOffset.UtcNow:R}\r\n" +
             "Content-Type: text/plain\r\n" +
-            "Content-Length: {asciiBody.Length}\r\n" +
+            $"Content-Length: {asciiBody.Length}\r\n" +
             "\r\n" +
-            "{asciiBody}\r\n";
+            $"{asciiBody}\r\n";
 
         private static Task ForCountAsync(int count, int dop, Func<int, Task> bodyAsync)
         {