Interoperability with gomock (#131)

This PR addresses two major issues
* `mockgen` cannot yet mock higher-level wrappers, like `*AndWait`
because of generics are not yet supported by gomock:
golang/mock#621
* fundamentally fixes #87

# Interoperability with `gomock`

When developing large applications, you find yourself in need of mocking
APIs. For Go, there's [`gomock`](https://github.com/golang/mock)
framework for code generating testing mocks. In this small example,
we'll show how to use `gomock` with Databricks SDK for Go.

Please read through
[`dbfs_test.go`](https://github.com/databricks/databricks-sdk-go/pull/131/files#diff-ac4ba8227927778705ac100663a6f04a314e590b0d0f19c1699fe63ae62b801e)
test example.

## Declaring which mocks to generate

```go
//go:generate go run github.com/golang/mock/mockgen@latest -package=mocks -destination=mocks/dbfs.go github.com/databricks/databricks-sdk-go/service/dbfs DbfsService
```

* `go run github.com/golang/mock/mockgen@latest` downloads and executes
the latest version of `mockgen` command
* `-package=mocks` instructs to generate mocks in the `mocks` package
* `-destination=mocks/dbfs.go` instructs to create `dbfs.go` file with
mock stubs.
* `github.com/databricks/databricks-sdk-go/service/dbfs` tells which
Databricks package to look services in.
* `DbfsService` tells which services to generate mocks for.

## Initializing `gomock`

Every test needs the following preamble:

```go
ctrl := gomock.NewController(t)
defer ctrl.Finish()
```

## Mocking individual methods with `gomock`

Every actual method call must be mocked for the test to pass:

```go
mockDbfs := mocks.NewMockDbfsService(ctrl)
mockDbfs.EXPECT().Create(gomock.Any(), gomock.Eq(dbfs.Create{
    Path:      "/a/b/c",
    Overwrite: true,
})).Return(&dbfs.CreateResponse{
    Handle: 123,
}, nil)
```

## Testing idioms with Databricks SDK for Go

You can stub out the auth with the `databricks.NewMockConfig(nil)`
helper function. Every service has a public property with the name of
the service plus `Service` suffix. You have to manually set the stubs
for every service that is called in unit tests.

```go
w := workspaces.New(databricks.NewMockConfig(nil))
w.Dbfs.DbfsService = mockDbfs
```

## Running this example

1. Run `go mod tidy` in this folder to create `go.sum` file to pick
dependency versions.
2. Run `go mod vendor` to download dependencies into `vendor/`
directory.
3. Run `go generate ./...` to create `mocks/` directory.
4. Run `go test ./...` to invoke tests with mocks.

README.md

@@ -71,7 +71,7 @@ During the Beta period, you must clone and then reference this repository locall
    )
    
    func init() {
-     databricks.WithProduct("your-product", "0.0.1")
+     databricks.WithProduct("awesome-product", "0.0.1")
    }
 
    func main() {
@@ -191,7 +191,7 @@ import (
 )
 
 func init() {
-  databricks.WithProduct("your-product", "0.0.1")
+  databricks.WithProduct("awesome-product", "0.0.1")
 }
 
 func main() {
@@ -274,7 +274,7 @@ import (
 )
 
 func init() {
-  databricks.WithProduct("your-product", "0.0.1")
+  databricks.WithProduct("awesome-product", "0.0.1")
 }
 
 func main() {
@@ -357,7 +357,7 @@ import (
 )
 
 func init() {
-  databricks.WithProduct("your-product", "0.0.1")
+  databricks.WithProduct("awesome-product", "0.0.1")
 }
 
 func main() {
@@ -426,7 +426,7 @@ import (
 )
 
 func init() {
-  databricks.WithProduct("your-product", "0.0.1")
+  databricks.WithProduct("awesome-product", "0.0.1")
 }
 
 func main() {
@@ -470,7 +470,7 @@ import (
 )
 
 func init() {
-  databricks.WithProduct("your-product", "0.0.1")
+  databricks.WithProduct("awesome-product", "0.0.1")
 }
 
 func main() {

databricks/mock.go

@@ -3,6 +3,11 @@ package databricks
 import "net/http"
 
 func NewMockConfig(auth func(r *http.Request) error) *Config {
+	if auth == nil {
+		auth = func(r *http.Request) error {
+			return nil
+		}
+	}
 	return &Config{
 		AuthType: "mock",
 		auth:     auth,