bug#42846: 27.1; [ruby-mode] setter symbol treated as assignment

[Nobuyoshi Nakada]

I found thatruby-mode.el in GNU Emacs 27.1 has a problem with the 
following code.


    :foo=
      # indent here

The setter symbol seems treated as an assignment

Also, the succeeding modifier is treated as the beginning of a block.

    :foo= if true
            # indent here