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

lib/core/dispatchRequest.js

@@ -4,8 +4,6 @@ var utils = require('./../utils');
 var transformData = require('./transformData');
 var isCancel = require('../cancel/isCancel');
 var defaults = require('../defaults');
-var isAbsoluteURL = require('./../helpers/isAbsoluteURL');
-var combineURLs = require('./../helpers/combineURLs');
 
 /**
  * Throws a `Cancel` if cancellation has been requested.
@@ -25,11 +23,6 @@ function throwIfCancellationRequested(config) {
 module.exports = function dispatchRequest(config) {
   throwIfCancellationRequested(config);
 
-  // Support baseURL config
-  if (config.baseURL && !isAbsoluteURL(config.url)) {
-    config.url = combineURLs(config.baseURL, config.url);
-  }
-
   // Ensure headers exist
   config.headers = config.headers || {};