Define string key for AxiosXHRConfigBase.headers (#11976)

When setting headers, tsc complains that "Index signature of object type implicitly has an 'any' type."
E.g. ajaxSettings.headers["Authorization"] = "secret";

axios/axios-tests.ts

@@ -69,6 +69,10 @@ axios.post("http://example.com/", {
     ]
 });
 
+var config: Axios.AxiosXHRConfigBase<any> = {headers: {}};
+config.headers['X-Custom-Header'] = 'baz';
+axios.post("http://example.com/", config);
+
 var getRepoIssue = axios.get<Issue>("https://api.github.com/repos/mzabriskie/axios/issues/1");
 
 var axiosInstance = axios.create({

axios/axios.d.ts

@@ -39,7 +39,7 @@ declare namespace Axios {
         /**
          * custom headers to be sent
          */
-        headers?: Object;
+        headers?: {[key: string]: any};
 
         /**
          * URL parameters to be sent with the request