Incorrect handling of commands with arguments (v3 unstable)

When using kingpin, I was expecting to be able to write the following code to parse a command:

```go
args := os.Args[1:]
switch kingpin.MustParse(app.Parse(args)) {
case initCmd.FullCommand():
initAction(*initCmdArg)
// ...
default:
app.Usage(args)
}
```

Given the CLI args `myapp init file.txt`, I got the usage message displayed, when I wanted initAction to be called.

Currently in kingpin (v3 unstable) I am working around this issue using:

```go
args := os.Args[1:]
switch kingpin.MustParse(app.Parse(args)) {
case "init":
initData(*initCmdArg)
// ...
default:
app.Usage(args)
}
```

## Expected Behavior

As stated above, I am expecting the output of `kingpin.MustParse(app.Parse(args))` to equal the result of `.FullCommand()` function of a command, rather than to have to define my own strings in the switch statement.

Below is an example block of test code that I would expect to passed.

```go
package main

import (
"testing"

kingpin "gopkg.in/alecthomas/kingpin.v3-unstable"
)

func TestParse(t *testing.T) {
app := kingpin.New("myApp", "Lorem ipsum.")
initCmd := app.Command("init", "Lorem init.")
_ = initCmd.Arg("file", "Lorem file.").String()

importCmd := app.Command("import", "Lorem import")

testCases := []struct {
testName string
args []string
want *kingpin.CmdClause
}{
{"SimpleCommand", []string{"import"}, importCmd},
{"CommandWithArg", []string{"init", "file.txt"}, initCmd},
}

for _, tc := range testCases {
t.Run(tc.testName, func(t *testing.T) {
parsed := kingpin.MustParse(app.Parse(tc.args))

if parsed != tc.want.FullCommand() {
t.Errorf("Expected %q to equal %q", parsed, tc.want.FullCommand())
}
})
}
}
```

## Current Behavior

Test Output:
```bash
Running tool: /usr/bin/go test -timeout 30s -tags -run ^TestParse$

--- FAIL: TestParse (0.00s)
--- FAIL: TestParse/CommandWithArg (0.00s)
/home/matthewdunsdon/go/src/github.com/matthewdunsdon/explore-kingpin-3/main_test.go:38: Expected "init" to equal "init []"
FAIL
exit status 1
FAIL github.com/matthewdunsdon/explore-kingpin-3 0.005s
Error: Tests failed.
```

I do not mind whether the parsed output is changed to `"init []"` or whether `importCmd.FullCommand()` returns `"init"`, however I would like the test to pass.

## Context

My reasons for using (v3 unstable) are that:

1. I want to be able to write apps where I can use the `app.UsageContext` to include examples in the usage context. I have written `github.com/matthewdunsdon/egcmd` [](https://godoc.org/github.com/matthewdunsdon/egcmd) and `github.com/matthewdunsdon/egcmd/egkingpin` [](https://godoc.org/github.com/matthewdunsdon/egcmd/egkingpin) to make this possible.
1. I have no intention to take the apps I write with (v3 unstable) into any production environment. I am using them for my own learning and own use.

## Environment
* Package source: `"gopkg.in/alecthomas/kingpin.v3-unstable"`
* Go version used: `1.8`
* Operating System and version (desktop or mobile): Solus (Linux x64)
```bash
> lsb_release -a
LSB Version: 1.4
Distributor ID: Solus
Description: Solus
Release: 2017.04.18.0
Codename: shannon
```