Fix can() so that it will nto die when called on a package

The Catalyst test body_fh.t was exploding because it gets this 'can'
method on it's object. The latest Test::Builder alpha also calls
->can(...) on test packages, in this case causing an error since the
package is not a hashref, but instead is just a string.

This will do the expected behavior for plack, but not explode if someone
calls 'can' on the package.

lib/Plack/Util.pm

@@ -309,7 +309,8 @@ package Plack::Util::Prototype;
 
 our $AUTOLOAD;
 sub can {
-    $_[0]->{$_[1]};
+    return $_[0]->{$_[1]} if Scalar::Util::blessed($_[0]);
+    goto &UNIVERSAL::can;
 }
 
 sub AUTOLOAD {