Share StreamCopyOperation implementations

[Tratcher]

https://github.com/aspnet/HttpAbstractions/blob/dev/src/Microsoft.AspNet.Http.Extensions/StreamCopyOperation.cs
https://github.com/aspnet/StaticFiles/blob/dev/src/Microsoft.AspNet.StaticFiles/StreamCopyOperation.cs

[glennc]

If StaticFiles already uses Http.Extensions then we should just use that location, otherwise find somewhere else for it to live.

Unify them when we do the work we already have scheduled to fix one of them.

[davidfowl]

Internal helper in common?

[BrennanConroy]

We can make it public in Http.Extensions since StaticFiles already depends on that and then we'll copy Ben's changes to the StaticFiles one over to Http.Extensions

[BrennanConroy]

Fixed via 3f84e99