[MODEL] Added the `update_document_attributes` method

Allows to directly perform a _partial_ update of a document, without consideration
for the changed attributes (as is the case in `update_document`)

Example:

    @article = Article.first
    @article.title = "New title"
    @article.__elasticsearch__.update_document_attributes({title: "New title"})

Closes #230
[MODEL] Small tweaks to the `update_document_attributes` method implementation

Related: #230
[MODEL] Improved the `test:all` Rake task for model