Adding no-format override to .NET Screenshot class for saving file

This commit adds a method to supply only a file name for the screenshot
image file, which will save the image in a Portable Network Graphics (PNG)
format.

Author: jimevans

dotnet/src/webdriver/Screenshot.cs

@@ -92,6 +92,17 @@ public byte[] AsByteArray
             get { return this.byteArray; }
         }
 
+        /// <summary>
+        /// Saves the screenshot to a Portable Network Graphics (PNG) file, overwriting the
+        /// file if it already exists.
+        /// </summary>
+        /// <param name="fileName">The full path and file name to save the screenshot to.</param>
+        /// to save the image to.</param>
+        public void SaveAsFile(string fileName)
+        {
+            this.SaveAsFile(fileName, ScreenshotImageFormat.Png);
+        }
+
         /// <summary>
         /// Saves the screenshot to a file, overwriting the file if it already exists.
         /// </summary>