Use explicit variable name over _

_ is generally used and intented for ignoring options. It allows for
having multiple _ parameters for example. The behavior of this is also
not consistent across at least Ruby 1.8 and 1.9, where on 1.8 it is the
value _, on 1.9 it works somewhat like a regular name.

To avoid all this confusion, this commit changes it to use a regular
name.