Write .NET screenshots using the Save overload that takes a FileStream

Rather than using the Image.Save overload that takes a string containing
the file name, we should use a System.IO.FileStream. The FileStream object
handles validating the existence of the path to which one is attempting to
write, so this will avoid the cryptic GDI+ error that occurs if the file
already exists. Additionally, the SaveAsFile method now does what it says
on the tin, that the file will be overwritten if it already exists. Fixes
issue #4645.

dotnet/src/webdriver/Screenshot.cs

@@ -119,15 +119,17 @@ public void SaveAsFile(string fileName, ScreenshotImageFormat format)
 
             using (MemoryStream imageStream = new MemoryStream(this.byteArray))
             {
-#if NETCOREAPP2_0 || NETSTANDARD2_0
                 using (FileStream fileStream = new FileStream(fileName, FileMode.Create))
-                {
+                { 
+#if NETCOREAPP2_0 || NETSTANDARD2_0
                     imageStream.WriteTo(fileStream);
-                }
 #else
-                Image screenshotImage = Image.FromStream(imageStream);
-                screenshotImage.Save(fileName, ConvertScreenshotImageFormat(format));
+                    using (Image screenshotImage = Image.FromStream(imageStream))
+                    {
+                        screenshotImage.Save(fileStream, ConvertScreenshotImageFormat(format));
+                    }
 #endif
+                }
             }
         }