Cleanup the error message with nullable types when invalid values are…

… used.

"Could not convert string `' to type Nullable`1..." is not nearly as helpful as
"Could not convert string `' to type Int32...".

	* src/NDesk.Options/NDesk.Options/Options.cs: Use the underlying
	  target type in the error message.
	* src/NDesk.Options/Test/NDesk.Options/OptionSetTest.cs: Add a test
	  for the underlying target type within the exception message.

src/NDesk.Options/NDesk.Options/Options.cs

@@ -360,7 +360,7 @@ protected static T Parse<T> (string value, OptionContext c)
 				throw new OptionException (
 						string.Format (
 							c.OptionSet.MessageLocalizer ("Could not convert string `{0}' to type {1} for option `{2}'."),
-							value, typeof (T).Name, c.OptionName),
+							value, targetType.Name, c.OptionName),
 						c.OptionName, e);
 			}
 			return t;

src/NDesk.Options/Test/NDesk.Options/OptionSetTest.cs

@@ -169,6 +169,9 @@ public void OptionalValues ()
 			Assert.AreEqual (n.Value, 42);
 			p.Parse (_("-n"));
 			Assert.AreEqual (n.HasValue, false);
+			Utils.AssertException (typeof(OptionException),
+					"Could not convert string `' to type Int32 for option `-n'.",
+					p, v => { v.Parse (_("-n=")); });
 		}
 
 		[Test]