Add more tests to should interface

Author: koddsson

test/should.js

@@ -339,6 +339,18 @@ describe('should', function() {
     }, "expected '' to be false")
   });
 
+  it('callable', function() {
+    (function() {}).should.be.callable;
+    (async function() {}).should.be.callable;
+    (function* () {}).should.be.callable;
+    (async function*() {}).should.be.callable;
+    true.should.not.be.callable;
+    
+    err(function(){
+      ''.should.be.callable;
+    }, "expected '' to be a callable function")
+  });
+
   it('null', function(){
     (0).should.not.be.null;