Encode dependency

LWP has a problem with older versions of Encode.  With Encode version
1.75 (which is the version included in a base perl-5.8.0), this test
failure occurs:

base/message..............NOK 99/118# Test 99 got: <UNDEF> (base/message.t at line 386)
#    Expected: "a\x{FFFD}"
#  base/message.t line 386 is: skip($NO_ENCODE, sub { $m->decoded_content }, "a\x{FFFD}");
base/message..............FAILED test 99                                     
        Failed 1/118 tests, 99.15% okay (less 11 skipped tests: 106 okay, 89.83%)

Empirically, Encode version 1.9801 (as found in a base perl-5.8.1)
does not produce this failure.  I don't think there's any need to probe
the versions in between; LWP could sanely reject all Encode versions
below 1.9801.

Unfortunately LWP can't just declare a dependency on Encode-1.9801,
because it doesn't actually require Encode at all.  But where Encode
is available, the version dependency ought to be handled in some
manner.  The two places that do "require Encode;" should probably add
"Encode->VERSION(1.9801);".  I'm not sure what could be done about
triggering an upgrade of an old Encode.  If upgrading isn't feasible,
the test suite could set $NO_ENCODE if Encode is of too low a version.

What do you think of these options?  I'm happy to produce a patch for
whatever approach is agreed on.

-zefram