Fix a crash when parsing the first header line that starts with a whitespace

Hello, I just discovered that Gmail always prepend a first line which contains only whitespaces when viewing message source. That line can crash node-imap's Parser, throwing this error:
```
TypeError: Cannot read property 'length' of undefined
    at parseHeader (/Users/top/wavify/node-imap/lib/Parser.js:991:26)
```

This PR fixes this issue by ignoring any folded line with no preceding line.