avoid converting objects twice

If I have an object of type T and want to set it as the default
value for a call to .keyAs(), I have to convert it to a string first.
This string would then get converted again in case the key doesn't exist
in the section.

initial.d

@@ -234,10 +234,10 @@ struct INISection
 	 + with a message containing location information.
 	 +/
 	T
-	keyAs(T)(string k, string defaultValue = null)
+	keyAs(T)(string k, T defaultValue)
 	{
 		try
-			return key(k, defaultValue).to!T();
+			return k in keys ? keys[k].to!T() : defaultValue;
 		catch (ConvException)
 			throw new INITypeException(
 				format!"unable to convert [%s].%s to %s"(name, k, T.stringof));