Inappropriate HttpClient set up for resumable downloads

[jskeet]

See googleapis/google-cloud-dotnet#824 for details, but basically we should be using HttpClientFactory. (I think it's reasonable to use the default one, given that the caller can always override the HttpClient in the ResumableUploadOptions.)

[jskeet]

(I'll create a patch for this shortly.)

[chrisdunelm]

Fixed via #930