Fixing combineURLs to support an empty relativeURL (#581)

* Fixing combineURLs to support an empty relativeURL

When combining the base and relative URLs, we should forego force
appending a slash to the base when the relative URL is empty.
This leads to a semantic url.

* Fixing combineURLs, allowing single slash relatives

test/specs/helpers/combineURLs.spec.js

@@ -12,4 +12,12 @@ describe('helpers::combineURLs', function () {
   it('should insert missing slash', function () {
     expect(combineURLs('https://api.github.com', 'users')).toBe('https://api.github.com/users');
   });
+
+  it('should not insert slash when relative url missing/empty', function () {
+    expect(combineURLs('https://api.github.com/users', '')).toBe('https://api.github.com/users');
+  });
+
+  it('should allow a single slash for relative url', function () {
+    expect(combineURLs('https://api.github.com/users', '/')).toBe('https://api.github.com/users/');
+  });
 });