fix(length): throw decent error on length check of null (#111)

Fixes #110

Author: AdriVanHoudt

lib/index.js

@@ -258,7 +258,7 @@ internals.addMethod('empty', internals.empty);
 
 internals.length = function (size) {
 
-    internals.assert(this, typeof this._ref === 'object' || typeof this._ref === 'string', 'Can only assert empty on object, array or string');
+    internals.assert(this, (typeof this._ref === 'object' && this._ref !== null) || typeof this._ref === 'string', 'Can only assert length on object, array or string');
 
     const length = this._ref.length !== undefined ? this._ref.length : Object.keys(this._ref).length;
     return this.assert(length === size, 'have a length of ' + size, length);

test/index.js

@@ -1538,6 +1538,20 @@ describe('expect()', () => {
                 Hoek.assert(exception.message === 'Expected \'a\' to have a length of 10 but got 1', exception);
                 done();
             });
+
+            it('throws on length check on objects with no length property', (done) => {
+
+                let exception = false;
+                try {
+                    Code.expect(null).to.have.length(2);
+                }
+                catch (err) {
+                    exception = err;
+                }
+
+                Hoek.assert(exception.message === 'Can only assert length on object, array or string', exception);
+                done();
+            });
         });
 
         describe('equal()', () => {