deal with missing closing boundary

@bf4 @jeremy
would you mind writing this in the /x style and adding comments? Like in https://github.com/mbleigh/acts-as-taggable-on/blob/ef7029dc456ec77c87e25ef38d69c695ac9ed67f/lib/acts_as_taggable_on/tag_list.rb#L23

```ruby
parts_regex = /
  (?:                    # non-capturing group
    \A                |  # start of string OR
    \r\n                 # line break
   )
  (
    --#{Regexp.escape(boundary || "")}  # boundary delimiter
    (?:--)?                                           # with optional closing
  )
  (?=\s*$)                                          # lookahead matching zero or more spaces followed by line-ending
  /x
```