Let the Doer Do it

[StevenACoffman]

This PR introduces switches to using the common interface Doer instead of the concrete *http.Client to be more flexible for consumers test construction.

Prior to this PR, genqlient has not supported one of the three common testing techniques by consumers. See Let the Doer Do it for more details.

---

Generally, there are three common ways to test HTTP client requests:

1. By Replacing http.Transport
   Transport specifies the mechanism by which individual HTTP requests are made. Instead of using the default http.Transport, we’ll replace it with our own implementation. To implement a transport, we’ll have to implement http.RoundTripper interface.
2. Using httptest.Server:
   httptest.Server allows us to create a local HTTP server and listen for any requests. When starting, the server chooses any available open port and uses that. So we need to get the URL of the test server and use it instead of the actual service URL.
3. Accept a Doer as a parameter instead of *http.Client
   The Doer is a single-method interface, as is often the case in Go:

type Doer interface {
    Do(*http.Request) (*http.Response, error)
}

Note: It does not really exist - it is not defined anywhere in the stdlib - but it is trivial to summon it from thin air in a package. Many people do as I did above, and because Go implicitly satisfies interfaces, any type that implements this method will be a valid Doer.

[benjaminjkraft]

Ok -- I'm not sure I agree that this is the best way to mock, but I don't think this is a place for me to be a stickler :-) .