#field_prefix/#prefix behavior broken, inconsistent with #field_suffix/#suffix in rendered addresses

From _addressfield_render_address() in addressfield.module:

      // Add #prefix and #field_suffix to the prefixes and suffixes.
      if (isset($child['#prefix'])) {
        $child['#prefix'] = (isset($child['#prefix']) ? $child['#prefix'] : '') . $child['#prefix'];
      }
      if (isset($child['#field_suffix'])) {
        $child['#suffix'] = (isset($child['#suffix']) ? $child['#suffix'] : '') . $child['#field_suffix'];
      }

The result is that fields are suffixed with both #suffix and #field_suffix as they may be set, but fields are never prefixed with #field_prefix. But if #prefix is set, then the field is prefixed with #prefix concatenated with itself!

The patch changes the prefix behavior to match the suffix behavior.