title: log2 issue when doing Survival Analysis
body: <p>I am planning to use TCGAbiolinks to perform an survival analysis by using the following code:</p>
  
  <pre><code>library(TCGAbiolinks)
  library(SummarizedExperiment)
  query_RNAseq &lt;- GDCquery(project = "TCGA-PAAD",
                           data.category = "Gene expression",
                           data.type = "Gene expression quantification",
                           platform = "Illumina HiSeq", 
                           file.type  = "results",
                           experimental.strategy = "RNA-Seq",
                           legacy = TRUE)
  GDCdownload(query_RNAseq, method = "api",chunks.per.download = 10) 
  PAADRnaseqSE &lt;- GDCprepare(query_RNAseq)
  
  # Survival Analysis SA
  clinical_patient_Cancer &lt;- GDCquery_clinic("TCGA-PAAD","clinical")
  dataPAADcomplete &lt;- log2(PAADRnaseqSE)
  </code></pre>
  
  <p>but it produced an error when I run the log2 function, code and error info are pasted below</p>
  
  <pre><code>&gt; dataPAADcomplete &lt;- log2(PAADRnaseqSE)
  Error in log(&lt;S4 object of class "RangedSummarizedExperiment"&gt;, 2) : 
    unused argument (2)
  </code></pre>
  
  <p>What should I do to solve the problem?</p>