src/ietf/rfc5234-core-abnf.pegjs: make HEXDIG strings case-insensitive

See http://tools.ietf.org/html/rfc5234#page-5:

```
   ABNF permits the specification of literal text strings directly,
   enclosed in quotation marks.  Hence:

         command     =  "command string"

   Literal text strings are interpreted as a concatenated set of
   printable characters.

   NOTE:

      ABNF strings are case insensitive and the character set for these
      strings is US-ASCII.

   Hence:

         rulename = "abc"

   and:

         rulename = "aBc"

   will match "abc", "Abc", "aBc", "abC", "ABc", "aBC", "AbC", and
   "ABC".
```