Fix override issue of default Block values

The default block values for HTML and YAML are both RegExp objects. These were getting caught by the _.isObject call, and thus were sent through _.assign. However, _.assign does not override RegExp objects. Therefore, it was not possible to override the default values.

Changing this to _.isPlainObject will cause RegExp objects to fall down to the else case, which will just assign the new value directly, fixing the issue.