Flush chunked writes before the entire response completes

[halter73]

Addresses #598

[halter73]

@davidfowl Should I even be looking at $DNX_PACKAGES anymore? Should I just do like you did in aspnet/dnx@a440f9a and get rid of the GetHome method?

         var profileDirectory = Environment.GetEnvironmentVariable("USERPROFILE");
         if (string.IsNullOrEmpty(profileDirectory))
         {
              profileDirectory = Environment.GetEnvironmentVariable("HOME");
         }

         return Path.Combine(profileDirectory, ".nuget", "packages");