Improve detection of HTTP "Range" requests in ConcurrentDownloadImage

[tri-adam]

The client supports parallel downloads using HTTP Range Requests. Currently, the client assumes ranged requests are supported if and only if the server performs a redirect via a http.StatusSeeOther. Otherwise, c.singleStreamDownload is utilized:

scs-library-client/client/pull.go

Lines 273 to 278 in 2e30d58

	if res.StatusCode == http.StatusOK {
	// Library endpoint does not provide HTTP redirection response, treat as single stream, direct download
	c.Logger.Logf("Library endpoint does not support concurrent downloads; reverting to single stream")
	return c.singleStreamDownload(ctx, dst, res, pb)
	}

This negatively impacts download performance against servers that support ranged requests, but do not redirect via http.StatusSeeOther, and is the root cause of #129.

The current logic should be replaced with logic that detects HTTP range support in the standard fashion (ref).

[EmmEff]

This currently affects (only) Singularity Enterprise 2.0.x, which does not redirect on the /v1/imagefile endpoint.