Make a single named param match @ sig in MAIN

Fixes R#2797.  This does **not** rewrite the signature binder.  Instead,
if the generated capture can not find any candidates, the capture changes
any named scalar values into 1 element arrays, and tries to find candidates
again.  If that is succesful, the modified signature is dispatched.

This is really a hack, but will in most cases do the right.

src/core/Main.pm6

@@ -266,6 +266,22 @@ my sub RUN-MAIN(&main, $mainline, :$in-as-argsfiles) {
         False
     }
 
+    sub find-candidates($capture) {
+        &main
+          # Get a list of candidates that match according to the dispatcher
+          .cando($capture)
+          # Sort out all that would fail due to binding
+          .grep: { !has-unexpected-named-arguments(.signature, $capture.hash) }
+    }
+
+    # turn scalar values of nameds into 1 element arrays, return new capture
+    sub scalars-into-arrays($capture) {
+        my %hash = $capture.hash.map: {
+            nqp::istype(.value,Positional) ?? $_ !! Pair.new(.key,[.value])
+        }
+        Capture.new( :list($capture.list), :%hash)
+    }
+
     # set up other new style dynamic variables
     my &*ARGS-TO-CAPTURE := &default-args-to-capture;
     my &*GENERATE-USAGE  := &default-generate-usage;
@@ -274,14 +290,17 @@ my sub RUN-MAIN(&main, $mainline, :$in-as-argsfiles) {
     my $capture := args-to-capture(&main, @*ARGS);
 
     # Get a list of candidates that match according to the dispatcher
-    my @matching_candidates = &main.cando($capture);
-
-    # Sort out all that would fail due to binding
-    @matching_candidates .=
-      grep: { !has-unexpected-named-arguments(.signature, $capture.hash) };
+    my @candidates = find-candidates($capture);
+    if !@candidates {
+        my $alternate = scalars-into-arrays($capture);
+        if find-candidates($alternate) -> @alternates {
+            $capture   := $alternate;
+            @candidates = @alternates;
+        }
+    }
 
     # If there are still some candidates left, try to dispatch to MAIN
-    if @matching_candidates {
+    if @candidates {
         if $in-as-argsfiles {
             my $*ARGFILES := IO::ArgFiles.new: (my $in := $*IN),
                 :nl-in($in.nl-in), :chomp($in.chomp), :encoding($in.encoding),