title: roxygenize fails with "missing name" for setAs()
body: <p>I am working on a package which defines its own S4 classes. I define a class and I want to define a method of converting objects of that class to a data frame. The code itself works, but an attempt to generate documentation with <code>roxygen2</code> warns as follows:</p>
  
  <pre><code>Warning: classes.R:14: Missing name
  </code></pre>
  
  <p>Here is a minimal code snippet:</p>
  
  <pre><code>#' The foo class
  #' 
  #' This is the foo class
  #' @slot .Data contains a list 
  foo &lt;- setClass("foo", slots=list(.Data="list"))
  
  #' @describeIn foo This is the method to show foo
  setMethod("show", "foo", 
    function(object) {
      cat("This is me, foo\n")
    })
  
  #' @describeIn foo This is the method to convert foo to a data frame
  setAs("foo", "data.frame", function(from) { 
      data.frame(cbind(from$a, from$b)) 
    })
  </code></pre>
  
  <p>The generated documentation does not contain any information about the <code>as(x, "data.frame")</code>. The following works without a warning:</p>
  
  <pre><code>#' The foo class
  #' 
  #' This is the foo class
  #' @rdname foo-class
  #' @slot .Data contains a list 
  foo &lt;- setClass("foo", slots=list(.Data="list"))
  
  #' @rdname foo-class
  setMethod("show", "foo",
    function(object) {
      cat("This is me, foo\n")
    })
  
  #' @name as
  #' @rdname foo-class
  setAs("foo", "data.frame", function(from) { data.frame(cbind(from$a, from$b)) })
  </code></pre>
  
  <p>..however, again, the documentation does not contain any information about the "as" idiom.</p>