Prevent memory leaks when cloning Range requests.

Prevent memory leaks when cloning Range requests.

HttpRequest::range field was set to a new HttpHdrRange object twice:
once in HttpRequest::clone() and once in HttpRequest::hdrCacheInit()
called from clone().

Polished HttpReply::clone() to make sure HttpReply::hdrCacheInit()
does not use uninitialized HttpReply::sline field and to prevent
benign double-initialization of HttpReply::keep_alive.