message: auto add mime_type
diff: #include "php_swoole_cxx.h"
  #include "swoole_http.h"
  +#include "mime_types.h"
  extern "C"
  {
  @@ -852,6 +853,13 @@ static PHP_METHOD(swoole_http_response, sendfile)
  swString *http_buffer = http_get_write_buffer(ctx);
  swString_clear(http_buffer);
  +
  + zval *zheader = sw_zend_read_and_convert_property_array(swoole_http_response_ce, ctx->response.zobject, ZEND_STRL("header"), 0);
  + if (!zend_hash_str_exists(Z_ARRVAL_P(zheader), ZEND_STRL("Content-Type")))
  + {
  + add_assoc_string(zheader, "Content-Type", swoole_mime_type_get(file));
  + }
  +
  http_build_header(ctx, http_buffer, length);
  if (!ctx->send(ctx, http_buffer->str, http_buffer->length))
  