Update http client testing section

I have remove all references to MockResponse::getRequestMethod as this was added in 5.2

Author: JohnstonCode

http_client.rst

@@ -1471,7 +1471,6 @@ Testing Request Data
 
 The ``MockResponse`` class comes with some helper methods to test the request:
 
-* ``getRequestMethod()`` - returns the HTTP method;
 * ``getRequestUrl()`` - returns the URL the request would be sent to;
 * ``getRequestOptions()`` - returns an array containing other information about
   the request such as headers, query parameters, body content etc.
@@ -1488,9 +1487,6 @@ Usage example::
         ],
     ]);
 
-    $mockResponse->getRequestMethod();
-    // returns "DELETE"
-
     $mockResponse->getRequestUrl();
     // returns "https://example.com/api/article/1337"
 
@@ -1567,7 +1563,6 @@ test it in a real application::
             $responseData = $service->createArticle($requestData);
 
             // Assert
-            self::assertSame('POST', $mockResponse->getRequestMethod());
             self::assertSame('https://example.com/api/article', $mockResponse->getRequestUrl());
             self::assertContains(
                 'Content-Type: application/json',