Fix issue when app.use() is called with empty value

lib/application.js

@@ -99,9 +99,9 @@ module.exports = class Application extends Emitter {
    */
 
   use(fn) {
-    debug('use %s', fn._name || fn.name || '-');
     if (typeof fn !== 'function') throw new TypeError('middleware must be a function!');
     if (isGeneratorFunction(fn)) throw new TypeError('Support for generators has been removed. See the documentation for examples of how to convert old middleware https://github.com/koajs/koa#example-with-old-signature');
+    debug('use %s', fn._name || fn.name || '-');
     this.middleware.push(fn);
     return this;
   }

test/application/use.js

@@ -54,11 +54,10 @@ describe('app.use(fn)', () => {
       .end(done);
   });
 
-  it('should throw error for non function', done => {
+  it('should throw error for non function', () => {
     const app = new Koa();
 
-    (() => app.use('not a function')).should.throw('middleware must be a function!');
-    done();
+    [null, undefined, 0, false, 'not a function'].forEach(v => (() => app.use(v)).should.throw('middleware must be a function!'));
   });
 
   it('should throw error for generator', () => {
@@ -67,10 +66,9 @@ describe('app.use(fn)', () => {
     (() => app.use(function *(){})).should.throw(/.+/);
   });
 
-  it('should throw error for non function', done => {
+  it('should throw error for non function', () => {
     const app = new Koa();
 
     (() => app.use('not a function')).should.throw('middleware must be a function!');
-    done();
   });
 });