Sanitize post types when adding meta boxes

[enrico-sorcinelli]

Recenlty, I added a meta box with a code like that:

$fm->add_meta_box( 'test meta box', '   post' );

by adding accidentally a blank space in the post type.
I notice that the box is still added, since behind the scenes, the type value is sanified.
However this is not be done when saving/updating post, so the custom fields are not updated.
I spent some time to understand why :-) so the patch try to solve that by sanitizing post_types values on constructors.

I had to check post_types type values since, even if the doc says that must be strings, many tests use
$field->add_meta_box( 'test meta box', $this->post ); and I think they should be fixed (it will simplify also this PR).
Note that command above doesn't fail and doesn't do anyting due to this.