fix: fix parsing of relative URLs

2.0.9 introduced a new URL parser based on WHATWG URL API. However, if
the request.url is relative (which is the case), parsing fails:
nodejs/node#12682

lib/url-parser.js

@@ -18,7 +18,7 @@ const Request = require('./request');
  */
 function httpToFloraRequest(httpRequest) {
     return new Promise((resolve, reject) => {
-        const parsedUrl = new URL(httpRequest.url);
+        const parsedUrl = new URL(httpRequest.url, 'http://localhost');
         const matches = parsedUrl.pathname.match(/^\/(.+)\/([^/.]*)(?:\.([a-z]+))?$/);
         if (!matches) {
             resolve(null);

test/url-parser.spec.js

@@ -47,6 +47,16 @@ describe('HTTP request parsing', () => {
             .catch(done);
     });
 
+    it('should parse relative urls', (done) => {
+        httpRequest.url = '/';
+        parseRequest(httpRequest)
+            .then((request) => {
+                expect(request).to.be.null;
+                done();
+            })
+            .catch(done);
+    });
+
     describe('flat resources', () => {
         it('should parse resource', (done) => {
             httpRequest.url = 'http://api.example.com/user/';