title: rbind list of data frames with one column of characters and numerics
body: <p>I have a list of two data frames with the same column names, but different number of rows, rbind.fill can help to put them together into a big data frame, but the problem is that the first column in df1 is numeric data, and df2 is character data, when they are merged, the character data all become 1, I've searched around, but didn't get the problem fixed, any help would be appreciated. A small example would be:</p>
  
  <pre><code>station &lt;- c(1:10)
  value &lt;- c(101:110)  
  df1 &lt;- data.frame(cbind(station,value)) 
  station &lt;- c("a","b")
  value &lt;- c(101:102)  
  df2 &lt;- data.frame(cbind(station,value)) 
  data1 &lt;- rbind.fill(df1,df2)
  </code></pre>
  
  <p>I would like the characters remain as characters, thanks.</p>