Return ErrResponseTooLarge instead of truncating response body

WithMaxResponseBody wrapped the body in io.LimitedReader, which returns EOF
at the cap, so Bytes/JSON/XML silently returned a truncated body with a nil
error despite the documented contract. Read one byte past the limit and
return the new ErrResponseTooLarge sentinel when exceeded; bodies exactly at
the limit still succeed.

client.go

@@ -102,9 +102,12 @@ func (c *Client) Do(ctx context.Context, req *http.Request) (*Response, error) {
 	}
 
 	if c.maxResponseBody > 0 {
+		// Read one byte past the limit so we can distinguish "exactly at the
+		// limit" (allowed) from "exceeds the limit" (ErrResponseTooLarge).
 		resp.Body = &limitedReadCloser{
-			R: io.LimitedReader{R: resp.Body, N: c.maxResponseBody},
-			C: resp.Body,
+			r:     io.LimitReader(resp.Body, c.maxResponseBody+1),
+			c:     resp.Body,
+			limit: c.maxResponseBody,
 		}
 	}