Allow supplying a custom text to status_header()

[nbachiyski]

Problem: I'd like to issue a HTTP 500 error, but include a text different from Internal Server Error, so that the client knows what caused the error.

Currently for a known error code the status_header() function always uses the global array $wp_header_to_desc and the only way to have my own text is to set the value in the global array.

The patch adds a second a argument to status_header().