Make Blob concatenation always return the left-hand type

Looking back at the history, this goes back until at least 2013 when
it was implemented as returning Buf if the types didn't match.

Other Blob related infixes return the type of the left-hand side,
so it makes sense to do this for concatentation as well.

In response to #5071

src/core.c/Buf.pm6

@@ -1423,7 +1423,7 @@ multi sub prefix:<~>(Blob:D \a) {
 
 multi sub infix:<~>(Blob:D $a) { $a }
 multi sub infix:<~>(Blob:D $a, Blob:D $b) {
-    my $res := nqp::create(nqp::eqaddr($a.WHAT,$b.WHAT) ?? $a !! Buf.^pun);
+    my $res := nqp::create($a);
     my $adc := nqp::decont($a);
     my $bdc := nqp::decont($b);
     my int $alen = nqp::elems($adc);