ETag assertion too restrictive

http://tools.ietf.org/html/rfc7232#section-2.3

```
     ETag       = entity-tag

     entity-tag = [ weak ] opaque-tag
     weak       = %x57.2F ; "W/", case-sensitive
     opaque-tag = DQUOTE *etagc DQUOTE
     etagc      = %x21 / %x23-7E / obs-text
                ; VCHAR except double quotes, plus obs-text
```

Currently only `/[a-z][0-9]+/i` is allowed.
