Attribute selector with pipe gets mangled #229

Context: cssnano/cssnano#925

console.log(require('postcss-selector-parser')().processSync('[ foo | bar ]'));
'[ foo  ]'

Expected output: [ foo | bar ].

Interestingly, it does work when removing some of the whitespace around the |:

• [ foo| bar ] => [ foo|bar ]
• [ foo|bar ] => [ foo|bar ]

But:

• [ foo |bar ] => [ foobar ]

Maybe related to #200