Fix bug when tag === false

[fstanis]

It seems there's an edge-case where another posthtml plugin produces (I believe valid) tree that looks like this:

{
  tag: false,
  attrs: {
    something: 'abc'
  },
  content: [
   ...
  ]
}

In this case, a placeholder element was replaced by changing tag to false, but the attrs property remained. This cases collapseBooleanAttributes to throw TypeError: node.tag.search is not a function since node.tag is a boolean and doesn't have the search property.

[maltsev]

Thanks for the PR, Filip!