[v2.0.1] space must exist after class name

[aleclarson]

~/pug-cli/node_modules/pug-lexer/index.js:60
    throw err;
    ^

Error: test.pug:59:7
    57| 
    58|     .123abc class name start with number
  > 59|     ._[invalid character in class name should result in it and text following be considered text
--------------^
    60| 
    61|     //  some html comment
    62|       p.

unexpected text "[inva"
    at makeError (~/pug-cli/node_modules/pug-error/index.js:32:13)
    at Lexer.error (~/pug-cli/node_modules/pug-lexer/index.js:59:15)
    at Lexer.fail (~/pug-cli/node_modules/pug-lexer/index.js:1441:10)
    at Lexer.advance (~/pug-cli/node_modules/pug-lexer/index.js:1501:15)
    at Lexer.callLexerFunction (~/pug-cli/node_modules/pug-lexer/index.js:1456:23)
    at Lexer.getTokens (~/pug-cli/node_modules/pug-lexer/index.js:1512:12)
    at lex (~/pug-cli/node_modules/pug-lexer/index.js:12:42)
    at Object.lex (~/pug-cli/node_modules/pug/lib/index.js:99:27)
    at Function.loadString [as string] (~/pug-cli/node_modules/pug-load/index.js:44:24)
    at compileBody (~/pug-cli/node_modules/pug/lib/index.js:86:18)