Fix incorrect keyword lexing

[schneems]

For some weird reason this line is detected as a keyword line:

            type: :module,

Though it clearly isn't. Ripper:

require 'ripper'

pp Ripper.lex(<<~'EOM')
  {
    type: :module,
  }
EOM

Produces:

[[[1, 0], :on_lbrace, "{", BEG|LABEL],
 [[1, 1], :on_ignored_nl, "\n", BEG|LABEL],
 [[2, 0], :on_sp, "  ", BEG|LABEL],
 [[2, 2], :on_label, "type:", ARG|LABELED],
 [[2, 7], :on_sp, " ", ARG|LABELED],
 [[2, 8], :on_symbeg, ":", FNAME],
 [[2, 9], :on_kw, "module", ENDFN],
 [[2, 15], :on_comma, ",", BEG|LABEL],
 [[2, 16], :on_ignored_nl, "\n", BEG|LABEL],
 [[3, 0], :on_rbrace, "}", END],
 [[3, 1], :on_nl, "\n", BEG],
 [[4, 0], :on_const, "EOM", CMDARG]]

This is the problem line:

 [[2, 9], :on_kw, "module", ENDFN],

Digging into the IRB source code they handled this case here ruby/ruby@776759e. Based on the description I believe this may be a bug in the lexer, but I'm not sure how to validate it.