Add withSession utility method

[thecodepixi]

WHAT is this pull request doing?

Adds withSession util/helper. This allows the user to pass in a set of parameters and receive back a client that is already hooked into the current active session (or the requested offline session), as well as the session itself.

For review, I would love extra eyes on my testing setup (all lines are covered but is there anything I should change or improve?).

Additionally, wondering if we should move this into the clients directory, as I think it would make sense to be able to call ShopifyAPI.Clients.withSession(<params>) from an end-user experience perspective.

[paulomarg]

Looks really good overall! Had a couple of minor comments but the logic looks solid to me.

[thecodepixi]

@paulomarg Any thoughts on the location of this code within the project? I think it would make a lot of sense to be able to call this off of Clients (example mentioned in my main PR comment), but I'm curious if you have any thoughts on that?

[paulomarg]

@paulomarg Any thoughts on the location of this code within the project? I think it would make a lot of sense to be able to call this off of Clients (example mentioned in my main PR comment), but I'm curious if you have any thoughts on that?

Hm, that's an interesting question - right now I see a bit of tension between Context and Utils, where each contains a bit of the public session interface, and we also have Shopify.Session. For now I say we keep this under Utils, but we take a closer look at what the public interface looks like now in a separate PR.

[paulomarg]

Looks great to me. Looking for the specific errors in the test makes it much easier to see what they are actually testing too, which is a nice bonus.