Fix 406 Client Error: Not Acceptable errors in test suite

There is a test case which validates that the HTTP client croaks
appropriately when connecting to non-CrateDB servers.

Within that test, we used http://crate.io to connect to. However, it
looks like Fastly now blocks excessive requests to that domain, for
example when running the test matrix on CI, which invokes quite a number
of requests.

> By default, the Signal Sciences agent returns a “406” response code when a request is blocked (similar to an HTTP 406 NOT ACCEPTABLE response).
>
> -- https://docs.fastly.com/signalsciences/faq/response-codes/#what-is-a-406-agent-response-code

Author: amotl

src/crate/client/doctests/http.txt

@@ -183,11 +183,11 @@ an exception is raised when the timeout is reached::
 
 When connecting to non Crate servers the HttpClient will raise a ConnectionError like this::
 
-    >>> http_client = HttpClient(["https://crate.io"])
+    >>> http_client = HttpClient(["https://httpbin.org/html"])
     >>> http_client.server_infos(http_client._get_server())
     Traceback (most recent call last):
     ...
-    crate.client.exceptions.ProgrammingError: Invalid server response of content-type 'text/html; charset=UTF-8':
+    crate.client.exceptions.ProgrammingError: Invalid server response of content-type 'text/html; charset=utf-8':
     ...
 
 When using the ``error_trace`` kwarg a full traceback of the server exception