net/http: expose "http: server gave HTTP response to HTTPS client" error

Expose "http: server gave HTTP response to HTTPS client" error as
ErrSchemeMismatch, so that it can be compared with errors.Is .

Fixes #44855

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

Author: AkihiroSuda

api/next/44855.txt

@@ -0,0 +1 @@
+pkg net/http, var ErrSchemeMismatch error #44855

src/net/http/client.go

@@ -204,6 +204,9 @@ func (c *Client) transport() RoundTripper {
 	return DefaultTransport
 }
 
+// ErrSchemeMismatch is returned when a server returns an HTTP response to an HTTPS client.
+var ErrSchemeMismatch = errors.New("http: server gave HTTP response to HTTPS client")
+
 // send issues an HTTP request.
 // Caller should close resp.Body when done reading from it.
 func send(ireq *Request, rt RoundTripper, deadline time.Time) (resp *Response, didTimeout func() bool, err error) {
@@ -265,7 +268,7 @@ func send(ireq *Request, rt RoundTripper, deadline time.Time) (resp *Response, d
 			// response looks like HTTP and give a more helpful error.
 			// See golang.org/issue/11111.
 			if string(tlsErr.RecordHeader[:]) == "HTTP/" {
-				err = errors.New("http: server gave HTTP response to HTTPS client")
+				err = ErrSchemeMismatch
 			}
 		}
 		return nil, didTimeout, err