docs: def get_headers(environ):
      """
      Returns only proper HTTP headers.
      """
      for key, value in environ.iteritems():
          key = str(key)
          if key.startswith('HTTP_') and key not in \
             ('HTTP_CONTENT_TYPE', 'HTTP_CONTENT_LENGTH'):
              yield key[5:].replace('_', '-').title(), value
          elif key in ('CONTENT_TYPE', 'CONTENT_LENGTH'):
              yield key.replace('_', '-').title(), value