title: Python email.header.decode_header fails for multiline headers
body: <p>I'm building a system that reads emails from a gmail account and fetches the subjects, using Python's <code>imaplib</code> and <code>email</code> modules. Sometimes, emails received from a hotmail account have line breaks in their headers, for instance:</p>
  
  <pre><code>In [4]: message['From']
  Out[4]: '=?utf-8?B?aXNhYmVsIG1hcsOtYSB0b2Npbm8gZ2FyY8OtYQ==?=\r\n\t&lt;isatocino22@hotmail.com&gt;'
  </code></pre>
  
  <p>If I try to decode that header, it does nothing:</p>
  
  <pre><code>In [5]: email.header.decode_header(message['From'])
  Out[5]: [('=?utf-8?B?aXNhYmVsIG1hcsOtYSB0b2Npbm8gZ2FyY8OtYQ==?=\r\n\t&lt;isatocino22@hotmail.com&gt;', None)]
  </code></pre>
  
  <p>However, if I replace the line break and tab with a space, it works:</p>
  
  <pre><code>In [6]: email.header.decode_header(message['From'].replace('\r\n\t', ' '))
  Out[6]: [('isabel mar\xc3\xada tocino garc\xc3\xada', 'utf-8'), ('&lt;isatocino22@hotmail.com&gt;', None)]
  </code></pre>
  
  <p>Is this a bug in <code>decode_header</code>? If not, I would like to know what other <em>special cases</em> like this I should be aware of.</p>