Wrong assumption of "content" field of hook_block_view output

In the function ctools_block_content_type_admin_info() (line 291, ctools/plugins/content_types/block/block.inc), it assumed that $block->content would be a string. This is a wrong assumption. According the API documentation page:

content: The content of the block's body. This may be a renderable array (preferable) or a string containing rendered HTML content. If the content is empty the block will not be shown.

When $block->content is a renderable array, the line 291 will cause fatal error: could not be converted to string in filter_xss(). Since Panels is using ctools for the interface, this bug has implications to other module. For example:

• #1983340: Recoverable fatal error: Object of class SiteActivity could not be converted to string in filter_xss() (line 1442 of /common.inc

To fix this, `$block->content` should be checked (and in some case rendered) before filter.

I've attached a patch to `ctools/plugins/content_types/block/block.inc`. The patch should fix the bug.