title: Julia: converting column type from Integer to Float64 in a DataFrame
body: <p>I am trying to change type of numbers in a column of a DataFrame from integer to floating point. It should be straightforward to do this, but it's not working. The data type remains to be integer. What am I missing?</p>
  
  <pre><code>In  [2]: using DataFrames
  df = DataFrame(A = 1:4, B = ["M", "F", "F", "M"])
  
  Out [2]: 4x2 DataFrame
  | Row | A | B   |
  |-----|---|-----|
  | 1   | 1 | "M" |
  | 2   | 2 | "F" |
  | 3   | 3 | "F" |
  | 4   | 4 | "M" |
  
  In  [3]: df[:,:A] = float64(df[:,:A])
  
  Out [3]: 4-element DataArray{Float64,1}:
   1.0
   2.0
   3.0
   4.0
  
  In  [4]: df
  
  Out [4]: 4x2 DataFrame
  | Row | A | B   |
  |-----|---|-----|
  | 1   | 1 | "M" |
  | 2   | 2 | "F" |
  | 3   | 3 | "F" |
  | 4   | 4 | "M" |
  
  In  [5]: typeof(df[:,:A])
  
  Out [5]: DataArray{Int64,1} (constructor with 1 method)
  </code></pre>