fix: do not trust spoofable Symbol.toStringTag in type dispatch

Every mode (src/index.js, src/full.js, src/lite.js, src/json.js)
picked its clone strategy from Object.prototype.toString.call(x),
which reads a user-settable Symbol.toStringTag. A plain object
declaring a recognized tag (eg "Map") was routed into that type's
clone branch and crashed on methods it doesn't have; one with an
unrecognized tag fell through every branch and was returned by
reference instead of cloned.

Also check the object's real constructor, which Symbol.toStringTag
cannot affect, so a plain object (or one with no prototype) is still
deep-cloned as a plain object no matter what tag it claims.

Fixes #54

test/json.js

@@ -10,5 +10,6 @@ suites.Nully(klona);
 
 suites.Objects(klona);
 suites.Arrays(klona);
+suites.ToStringTag(klona);
 
 suites.Pollutions(klona);