Touch Invalid Parameters Error: Detected on customer upgrades

Request
 
Modify the API or documentation for Touch function to don't ask for document's CAS parameter, touch doesn't amend document value.

https://godoc.org/github.com/couchbase/gocb#Bucket.Touch

 
Background
 
 The following snippet was accepted when working with to 4.6 version:

``` 
fragment, mErr := bucket.MutateIn("1", 0, 0).Upsert("name", "New Test", false).Execute()
    if mErr != nil {
        logrus.Error("Mutate " + mErr.Error())
    }

    _, tErr := bucket.Touch("1", gocb.Cas(fragment.Cas()), uint32(20))
```
 Problem
 
When you execute the same code on 5.x server an invalid error parameter is raised, the workaround is to fix the CAS value to 0:

```
    _, tErr := bucket.Touch("1", gocb.Cas(0), uint32(20))
```