Should IP6Address(IPAddress) actually construct IPv4 Mapped address?

Hi again,

The constructor IP6Address(IPAddress) just zeroes IPv6 address and puts
the IPv4 on lower 4 octets (the result is called "IPv4 compatible
address", use of which is deprecated -- see RFC-4291 2.5.5.1).

Would it break anything if this constructor actually created
an usable address using "IPv4 mapped address" - RFC-4291 2.5.5.2)?

Especially, when the function "IP6Address::ip4_address" returns true
only for ipv4-mapped address.

[Sorry about just sending these mails without actual patch proposals,
but I may eventually get into that too in a bit.. ]