Dir::Tmpname.make_tmpname inconsistent with Ruby 2.2+

In Ruby 2.1 you had to do:

``` ruby
Dir::Tmpname.make_tmpname(["prefix", ""], nil)
Dir::Tmpname.make_tmpname(["prefix", ".ext"], nil)
Dir::Tmpname.make_tmpname(["", ".ext"], nil)
```

In 2.2+ the second value of the array is optional so you can do:

``` ruby
Dir::Tmpname.make_tmpname(["prefix"], nil)
```

if you do not wish to have an extension.
