Fix encoding IPv6 addresses in pvalues

[aszlig]

RFC 8601 section 2.2 specifies the pvalue field like this:

  pvalue = [CFWS] ( value / [ [ local-part ] "@" ] domain-name )
           [CFWS]
         ; the value extracted from the message property defined
         ; by the "ptype.property" construction

The value here refers to RFC 2045 section 5.1, here are the relevant parts:

  value := token / quoted-string
  token := 1*<any (US-ASCII) CHAR except SPACE, CTLs,
              or tspecials>

  tspecials :=  "(" / ")" / "<" / ">" / "@" /
                "," / ";" / ":" / "\" / <">
                "/" / "[" / "]" / "?" / "="
                ; Must be in quoted-string,
                ; to use within parameter values

Since IPv6 addresses contain : characters that are included in tspecials above, it needs to be a quoted-string.

[mdecimus]

Good catch! To avoid an extra allocation please create a wrapper struct that implements Display and renders the IP using format.

[aszlig]

To avoid an extra allocation please create a wrapper struct that implements Display and renders the IP using format.

Okay, will avoid that allocation. However, I think IMHO a newtype is only warranted if it's something that is more generic (and even then I think it's questionable) and using something like IpAddressAsPvalue sounds a bit too excessive to me, so I'll change the function instead.