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/adapters/http.js

@@ -2,6 +2,7 @@
 
 var utils = require('./../utils');
 var settle = require('./../core/settle');
+var buildFullPath = require('../core/buildFullPath');
 var buildURL = require('./../helpers/buildURL');
 var http = require('http');
 var https = require('https');
@@ -64,7 +65,8 @@ module.exports = function httpAdapter(config) {
     }
 
     // Parse url
-    var parsed = url.parse(config.url);
+    var fullPath = buildFullPath(config.baseURL, config.url);
+    var parsed = url.parse(fullPath);
     var protocol = parsed.protocol || 'http:';
 
     if (!auth && parsed.auth) {