fix Function1Comonad#cojoin

old implementation does not satisfy `cobind(identity) === cojoin`

```scala
scala> import scalaz._,Scalaz._
import scalaz._
import Scalaz._

scala> val f = {x: Int => x + 1}
f: Int => Int = <function1>

scala> val f1 = f.cojoin
f1: Int => (Int => Int) = <function1>

scala> val f2 = f.cobind(identity)
f2: Int => (Int => Int) = <function1>

scala> f1(2)(3)
res0: Int = 5

scala> f2(2)(3)
res1: Int = 6
```