ListGroupProjectOptions should be created as type ListProjectOptions (#408)

* Fixes https://github.com/xanzy/go-gitlab/issues/407
* Add the first groups unit test

groups.go

@@ -239,13 +239,13 @@ func (s *GroupsService) SearchGroup(query string, options ...OptionFunc) ([]*Gro
 // options.
 //
 // GitLab API docs:
-// https://docs.gitlab.com/ce/api/groups.html#list-a-group-s-projects
-type ListGroupProjectsOptions ListOptions
+// https://docs.gitlab.com/ce/api/groups.html#list-a-group-39-s-projects
+type ListGroupProjectsOptions ListProjectsOptions
 
 // ListGroupProjects get a list of group projects
 //
 // GitLab API docs:
-// https://docs.gitlab.com/ce/api/groups.html#list-a-group-s-projects
+// https://docs.gitlab.com/ce/api/groups.html#list-a-group-39-s-projects
 func (s *GroupsService) ListGroupProjects(gid interface{}, opt *ListGroupProjectsOptions, options ...OptionFunc) ([]*Project, *Response, error) {
 	group, err := parseID(gid)
 	if err != nil {

groups_test.go

+package gitlab
+
+import (
+	"fmt"
+	"net/http"
+	"reflect"
+	"testing"
+)
+
+func TestListGroupProjects(t *testing.T) {
+	mux, server, client := setup()
+	defer teardown(server)
+
+	mux.HandleFunc("/api/v4/groups/22/projects",
+		func(w http.ResponseWriter, r *http.Request) {
+			testMethod(t, r, "GET")
+			fmt.Fprint(w, `[{"id":1},{"id":2}]`)
+		})
+
+	opt := &ListGroupProjectsOptions{
+		ListOptions: ListOptions{2, 3},
+		Archived:    Bool(true),
+		OrderBy:     String("name"),
+		Sort:        String("asc"),
+		Search:      String("query"),
+		Simple:      Bool(true),
+		Visibility:  Visibility(PublicVisibility),
+	}
+
+	projects, _, err := client.Groups.ListGroupProjects(22, opt)
+	if err != nil {
+		t.Errorf("Groups.ListGroupProjects returned error: %v", err)
+	}
+
+	want := []*Project{{ID: 1}, {ID: 2}}
+	if !reflect.DeepEqual(want, projects) {
+		t.Errorf("Groups.ListGroupProjects returned %+v, want %+v", projects, want)
+	}
+}