title: Two arrays of conditions in single ActiveRecord 'where' clause
body: <p>Let's say I have two pairs of SQL conditions like these:</p>
  
  <pre><code>a = [ "users.accepted = ? AND users.active_at &gt; ?", true,  Time.zone.now ]
  b = [ "users.accepted = ? AND users.active_at &gt; ?", false, Time.zone.now + 3.days ]
  </code></pre>
  
  <p>I can use code like <code>User.where(a)</code> to get all rows that satisfy the <code>a</code> condition. How can I use <code>where</code> to get rows that satisfy either <code>a</code> or <code>b</code> conditions? The result should be <code>ActiveRecord::Relation</code>.</p>