Use https:// in examples in README.md (#280)

Author: jonasfj

README.md

@@ -15,12 +15,12 @@ you to make individual HTTP requests with minimal hassle:
 ```dart
 import 'package:http/http.dart' as http;
 
-var url = 'http://example.com/whatsit/create';
+var url = 'https://example.com/whatsit/create';
 var response = await http.post(url, body: {'name': 'doodle', 'color': 'blue'});
 print('Response status: ${response.statusCode}');
 print('Response body: ${response.body}');
 
-print(await http.read('http://example.com/foobar.txt'));
+print(await http.read('https://example.com/foobar.txt'));
 ```
 
 If you're making multiple requests to the same server, you can keep open a
@@ -30,7 +30,7 @@ If you do this, make sure to close the client when you're done:
 ```dart
 var client = new http.Client();
 try {
-  var uriResponse = await client.post('http://example.com/whatsit/create',
+  var uriResponse = await client.post('https://example.com/whatsit/create',
       body: {'name': 'doodle', 'color': 'blue'});
   print(await client.get(uriResponse.bodyFields['uri']));
 } finally {