Do not modify config.url when using a relative baseURL (resolves #1628) (#2391)

* Adding tests to show config.url mutation

Because config.url is modified while processing the request
when the baseURL is set,
it is impossible to perform a retry with the provided config object.

Ref #1628

* Fixing url combining without modifying config.url

As config.url is not modified anymore during the request processing.
The request can safely be retried after it failed with the provided
config.

resolves #1628

test/unit/adapters/http.js

@@ -646,6 +646,20 @@ describe('supports http with nodejs', function () {
       });
     });
   });
+
+  it('should combine baseURL and url', function (done) {
+    server = http.createServer(function (req, res) {
+      res.end();
+    }).listen(4444, function () {
+      axios.get('/foo', {
+        baseURL: 'http://localhost:4444/',
+      }).then(function (res) {
+        assert.equal(res.config.baseURL, 'http://localhost:4444/');
+        assert.equal(res.config.url, '/foo');
+        done();
+      });
+    });
+  });
 });