Describe the bug
When converting a classic post that contained a short-code generated by the NextGen Gallery plugin, two blocks are created: a short-code block containing part of the short code, and a text block containing the remaining portion of the short-code.

To Reproduce
Steps to reproduce the behavior:

1. Create a classic post with the following short code contained in HTML:
   <p>[ngg_images source="galleries" container_ids="2" display_type="photocrati-nextgen_basic_thumbnails" override_thumbnail_settings="0" thumbnail_width="240" thumbnail_height="160" thumbnail_crop="1" images_per_page="20" number_of_columns="0" ajax_pagination="1" show_all_in_lightbox="0" use_imagebrowser_effect="0" show_slideshow_link="1" slideshow_link_text="[Show slideshow]" order_by="sortorder" order_direction="ASC" returns="included" maximum_entity_count="500"]</p>
2. Click on "Convert to Blocks"
3. Note that while a short code block was created, a text block was also created containing:
   " order_by="sortorder" order_direction="ASC" returns="included" maximum_entity_count="500"]

Expected behavior
A single short-code block should be created.

Screenshots

HTML prior to converting to blocks:

Visual blocks after conversion:

Desktop:

• OS: Mac OSX 10.13.6
• Browser: Safari
• Version: 11.1.2

Additional context

• Please add the version of Gutenberg you are using in the description.
• To report a security issue, please visit the WordPress HackerOne program: https://hackerone.com/wordpress.