Str.comb takes its cue from string rather than numeric value

Fixes GH#2010.

In GH#1552 it was decided that the boolean value of allomorphs is decided by
the numerical value, not by the string value.  So from that, it *could* follow
that the behaviour is correct.  However, since `.comb` clearly works on the
the string part of the allomorph, it seemed like the right thing to do to use
the string semantics of Bool instead in this case.

src/core/Str.pm6

@@ -352,7 +352,7 @@ my class Str does Stringy { # declared in BOOTSTRAP
             }
             method new(\string) {
                 nqp::if(
-                  string,
+                  string.chars,  # GH#1020
                   nqp::create(self)!SET-SELF(string),
                   Rakudo::Iterator.Empty
                 )