title: Ruby code to get the date of next Monday (or any day of the week)
body: <p>Given an input of, for example,</p>
  <pre><code>day = 'Monday'
  </code></pre>
  <p>how can I calculate the date of the next <code>day</code>?</p>
  <pre><code>def date_of_next(day)
    ...
  end
  </code></pre>