Request body: removed error assumption (ticket #2058).

Request body: removed error assumption (ticket #2058).

Before introduction of request body filter in 42d9beeb22db, the only
possible return code from the ngx_http_request_body_filter() call
without actual buffers was NGX_HTTP_INTERNAL_SERVER_ERROR, and
the code in ngx_http_read_client_request_body() hardcoded the only
possible error to simplify the code of initial call to set rb->rest.

This is no longer true after introduction of request body filters though,
as a request body filter might need to return other errors, such as 403.
Fix is to preserve the error code actually returned by the call
instead of assuming 500.