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/specs/core/buildFullPath.spec.js

@@ -0,0 +1,20 @@
+var buildFullPath = require('../../../lib/core/buildFullPath');
+
+describe('helpers::buildFullPath', function () {
+  it('should combine URLs when the requestedURL is relative', function () {
+    expect(buildFullPath('https://api.github.com', '/users')).toBe('https://api.github.com/users');
+  });
+
+  it('should return the requestedURL when it is absolute', function () {
+    expect(buildFullPath('https://api.github.com', 'https://api.example.com/users')).toBe('https://api.example.com/users');
+  });
+
+  it('should not combine URLs when the baseURL is not configured', function () {
+    expect(buildFullPath(undefined, '/users')).toBe('/users');
+  });
+
+  it('should combine URLs when the baseURL and requestedURL are relative', function () {
+    expect(buildFullPath('/api', '/users')).toBe('/api/users');
+  });
+
+});