Fix multiline parsing without format_firstline

`format multiline` in in_tail plugin without `format_firstline` option does not work. 
In multiline mode, record is emitted only when input line is matched to regex, but the input line passed to regex is always single line, so it never match without `format_firstline` option.
Input line is handled [here](https://github.com/fluent/fluentd/blob/06d176bd00f345d227a7f34d0aa86bd874cf3dc7/lib/fluent/plugin/in_tail.rb#L260-L271) and regex is set [here](https://github.com/fluent/fluentd/blob/06d176bd00f345d227a7f34d0aa86bd874cf3dc7/lib/fluent/parser.rb#L388).

I don't know `format_firstline` is required.  What case the option is better for?