URL-encode path parameters for Router.url

The main breaking change from v8.x to v9.x was an upgrade to `path-to-regex`. [That PR](#71) alluded to a breaking change in encoding. Namely, parameters were not URL-encoded: parameters with safe special characters (like spaces) were not percent-encoded, and parameters with special characters that mean something in a URL, such as slashes (path separators) and question marks (query string delimiter).

The motivation for this PR is to make URL-encoding be the default since typically the parameters provided to `Router.url` are plain, unencoded values. Should someone need an escape hatch, they could pass in `{ encode: null }` (I think) to disable the automatic encoding.

Updated tests and docs.

API.md

@@ -424,7 +424,7 @@ router
 
 ### Router.url(path, params) ⇒ <code>String</code>
 
-Generate URL from url pattern and given `params`.
+Generate URL from url pattern and given `params`. This method URL-encodes the parameters before including them in the URL.
 
 **Kind**: static method of <code>[Router](#exp_module_koa-router--Router)</code>
 

lib/layer.js

@@ -119,7 +119,7 @@ Layer.prototype.url = function (params, options) {
     }
   }
 
-  const toPath = compile(url, options);
+  const toPath = compile(url, Object.assign({ encode: encodeURIComponent }, options));
   let replaced;
 
   const tokens = parse(url);

test/lib/layer.js

@@ -303,6 +303,12 @@ describe('Layer', function () {
       url.should.equal('/programming/how-to-node');
     });
 
+    it('escapes using encodeURIComponent()', function() {
+      const route = new Layer('/:category/:title', ['get'], [function () {}], {name: 'books'});
+      const url = route.url({ category: 'programming', title: 'how to node & js/ts' });
+      url.should.equal('/programming/how%20to%20node%20%26%20js%2Fts');
+    });
+
     it('setPrefix method checks Layer for path', function () {
       const route = new Layer('/category', ['get'], [function () {}], {
         name: 'books'

test/lib/router.js

@@ -1674,6 +1674,14 @@ describe('Router', function () {
       router.url('Picard').should.be.Error();
       router.url(Symbol('books')).should.be.Error();
     });
+
+    it('escapes using encodeURIComponent()', function() {
+      const url = Router.url(
+        '/:category/:title',
+        { category: 'programming', title: 'how to node & js/ts' }
+      );
+      url.should.equal('/programming/how%20to%20node%20%26%20js%2Fts');
+    });
   });
 
   describe('Router#param()', function () {