title: Why are NA values inserted to postgresql not getting type casted to NULL?
body: <p>I have a line of code something like this:</p>
  
  <pre><code>query &lt;- "INSERT INTO my_books (title, subtitle, content) VALUES ($1,$2,$3)"
  data_to_be_inserted &lt;- c("hello", NA, "world")
  dbExecute(db_connection_to_postgresql, query, data_to_be_inserted)
  </code></pre>
  
  <p>When doing this, the values in the db are:</p>
  
  <pre><code>SELECT * FROM my_books;
  
  id title    subtitle  content
  1  "hello"  "NA"      "world"
  </code></pre>
  
  <p>instead of:</p>
  
  <pre><code>SELECT * FROM my_books;
  
  id title    subtitle  content
  1  "hello"  NULL      "world"
  </code></pre>
  
  <p>Why is this?</p>