Classes array not initialized (Picture module template)

Hi loparr,

No sure if the $variables['attributes']['class'] is expected to be a string or array elsewhere.
I also found it gave out warnings if the $variables['attributes']['class'] was missing,

updated /sites/all/themes/bootstrap/templates/picture/picture.vars.php
as follow and it then worked as expected:

function bootstrap_preprocess_picture(&$variables) {
  // Add responsiveness, if necessary.
  if ($shape = bootstrap_setting('image_responsive')) {
    if (empty($variables['attributes']['class'])) {
      $variables['attributes']['class'] = array();
    }
    if (!is_array($variables['attributes']['class'])) {
      $variables['attributes']['class'] .= ' img-responsive';
    }
    else {
      $variables['attributes']['class'][] = 'img-responsive';
    }
  }
}

This actually looks like a Bootstrap issue instead of a Picture issue (the theme needs to support the module).

We need to initialize an array if it hasn't been already.

Rolled the last suggestion into a patch.

This is not an issue with this project.