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/buildFullPath.js

@@ -0,0 +1,20 @@
+'use strict';
+
+var isAbsoluteURL = require('../helpers/isAbsoluteURL');
+var combineURLs = require('../helpers/combineURLs');
+
+/**
+ * Creates a new URL by combining the baseURL with the requestedURL,
+ * only when the requestedURL is not already an absolute URL.
+ * If the requestURL is absolute, this function returns the requestedURL untouched.
+ *
+ * @param {string} baseURL The base URL
+ * @param {string} requestedURL Absolute or relative URL to combine
+ * @returns {string} The combined full path
+ */
+module.exports = function buildFullPath(baseURL, requestedURL) {
+  if (baseURL && !isAbsoluteURL(requestedURL)) {
+    return combineURLs(baseURL, requestedURL);
+  }
+  return requestedURL;
+};