title: R hash a column does not work using digest
body: <p>I have a dataframe that contains a column which is the unique identifier and I will like to create a new column which is the hash value. 
  I am using the transform from dplyr and digest for hashing but looks like all the hashed value are the same. </p>
  
  <pre><code>          status  identifier
  1         NEW     1035656|8000|157.6|2018-12-10 00:00:00.0|2018-12-06 00:00:00.0
  2         NEW     1852231|460000|1748.0|2018-03-09 00:00:00.0|2018-03-07 00:00:00.0
  3         NEW     3197282|6000|55.2|2019-01-18 00:00:00.0|2019-01-16 00:00:00.0
  4         NEW     1827398|396000|21859.2|2019-02-25 00:00:00.0|2019-02-21 00:00:00.0
  5         NEW     1148967|60000|150.0|2018-10-15 00:00:00.0|2018-10-11 00:00:00.0
  </code></pre>
  
  <p>Output</p>
  
  <pre><code>data_new &lt;- transform(data, hash=digest(identifier, algo="md5", serialize = F))
  </code></pre>
  
  <p>And the outcome looks wrong.  </p>
  
  <pre><code>hash
  1 d1ede7da2094651658adfd6171c33c52
  2 d1ede7da2094651658adfd6171c33c52
  3 d1ede7da2094651658adfd6171c33c52
  4 d1ede7da2094651658adfd6171c33c52
  5 d1ede7da2094651658adfd6171c33c52
  6 d1ede7da2094651658adfd6171c33c52
  </code></pre>
  
  <p>Can someone explain to me what is wrong with me syntax? </p>