Fix HTTP status code modification while exception handling (#2027)

[tanakahisateru]

Fixes #2027

The headers_sent() guard does not prevent the E_WARNING from http_response_code(). The warning is emitted when header('HTTP/...') has already been called, but headers_sent() only becomes true after output is flushed to the stream — these are independent states.

headers_list() might seem like an alternative to detect a previously set status line, but it intentionally excludes the HTTP status line.

Dropping the headers_sent() check and suppressing the warning with @ is the most correct approach available: it sets 500 when possible and silently does nothing when it cannot.

[Seldaek]

I see the value of adding the @, but why remove the check? Sounds to me like it's still better to avoid creating an E_WARNING if we can (i.e. if headers are sent we know it is going to error so might as well skip it).