Tweaked ExpressionStringBuilder to support Xor and Coalesce operators

This is a temporary solution.

I will create a pull request to ExpressionToString and then update the
package here.

TestStack.FluentMvcTesting/App_Packages/ExpressionStringBuilder.0.9.2/ExpressionStringBuilder.cs

@@ -266,6 +266,10 @@ private static string ToString(ExpressionType type)
                     return "||";
                 case ExpressionType.Subtract:
                     return "-";
+                case ExpressionType.Coalesce:
+                    return "??";
+                case ExpressionType.ExclusiveOr:
+                    return "^";
                 default:
                     throw new NotImplementedException();
             }