Allow empty lines in htdigest files

In some cases, the line can be an empty string (not a whitespace character), which the given regular expression does not match.

I'd also like some advice on a related issue: any malformed htdigest file leads to a crash without a reasonable error message. The Exc.jsDAV_Exception ends up passed to abstractDigest's requireAuth() method, which attempts to call addHeader() on it. Stack trace:

```
jsDAV/lib/DAV/plugins/auth/abstractDigest.js:226
        err.addHeader("WWW-Authenticate", "Digest realm=\"" + realm + 
            ^
TypeError: Object undefined has no method 'addHeader'
    at Object.module.exports.jsDAV_Auth_iBackend.extend.requireAuth (jsDAV/lib/DAV/plugins/auth/abstractDigest.js:226:13)
    at module.exports.jsDAV_Auth_iBackend.extend.authenticate (jsDAV/lib/DAV/plugins/auth/abstractDigest.js:254:29)
    at next (jsDAV/lib/DAV/plugins/auth/file.js:89:24)
    at module.exports.jsDAV_Auth_Backend_AbstractDigest.extend.loadFile (jsDAV/lib/DAV/plugins/auth/file.js:58:28)
```
Would guarding that addHeader call with a check that err is an instanceof Exc.NotAuthenticated be sufficient? I need to read more of the code to figure out what's supposed to happen here.