title: R, xgboost: label must be in [0,1] for logistic regression
body: <p>I am getting an error saying that my label must be in [0, 1]:</p>
  
  <pre><code>&gt; system.time(xgb &lt;- xgboost(params  = param,
  +                            data    = dtrain,
  +                            label   = as.numeric(train.label),
  +                            nrounds = 500,
  +                            print_every_n = 100,
  +                            verbose = 1))
  Error in xgb.iter.update(bst$handle, dtrain, iteration - 1, obj) : 
    [10:39:29] amalgamation/../src/objective/regression_obj.cc:108: label must be in [0,1] for logistic regression
  Timing stopped at: 0.11 0 0.11
  </code></pre>
  
  <p>However, my label is in [0, 1]:</p>
  
  <pre><code>&gt; train.label
     [1] 1 1 1 1 1 0 0 1 1 0 0 1 1 1 1 0 1 1 1 1 1 0 1 1 . . .
   [ reached getOption("max.print") -- omitted 38907 entries ]
  Levels: 0 1
  </code></pre>
  
  <p>I also tried converting to a <code>numeric</code> data type without luck.</p>