Added special character handling to StringConverter

Added special character handling for FormFeed, Backspace, Backslash, and Tab

Author: mizady

nanoFramework.Json/Converters/StringConverter.cs

@@ -15,6 +15,10 @@ internal sealed class StringConverter : IConverter
         {
             {'\n', 'n'},
             {'\r', 'r'},
+            {'\b', 'b' },
+            {'\f', 'f' },
+            {'\t', 't' },
+            {'\\', '\\' },
             {'\"', '"' }
         };