Check for duplicate Content-Length headers in requests

If a duplicate CL header is in the request, we fail the request with a
400 (Bad Request)

Fix a test case that was sending duplicate CL by misstake and would
not fail because of that.
http_copyh(const struct http *to, const struct http *fm, unsigned n)
{

	assert(n < HTTP_HDR_FIRST);
	Tcheck(fm->hd[n]);
	to->hd[n] = fm->hd[n];
	to->hdf[n] = fm->hdf[n];
}

http_copyh(const struct http *to, const struct http *fm, unsigned n)
{

	assert(n < HTTP_HDR_FIRST);
	Tcheck(fm->hd[n]);
	to->hd[n] = fm->hd[n];
	to->hdf[n] = fm->hdf[n];
}
