Fix a problem with omitting the second argument of UnitOf

[hadashiA]

refs: #33

In the case of omitting the 2nd argument of UnitOf with set of any named properties, there is unintented behavior.

Like this:

[UnitOf(typeof(string),  ToStringFormat = "...")]
public partial struct Foo

Fix:
The name of the argument is determined first before the number of the argument..

[neuecc]

thx