add support for GET user/activities #248

Since the response of `GET user/activities` contains a date format (`YYYY-MM-DD`) that cannot be unmarshalled into a `time.Time` automatically, additional logic was added to to cover the un-marshalling of the timestamps:

`UserActivityTime.UnmarshalJSON()`
`UserActivityTime.MarshalJSON()`
`UserActivityTime.IsSet()`

see https://stackoverflow.com/questions/25087960/json-unmarshal-time-that-isnt-in-rfc-3339-format

users.go

@@ -20,6 +20,7 @@ import (
 	"errors"
 	"fmt"
 	"time"
+	"strings"
 )
 
 // UsersService handles communication with the user related methods of
@@ -696,3 +697,67 @@ func (s *UsersService) RevokeImpersonationToken(user, token int, options ...Opti
 
 	return s.client.Do(req, nil)
 }
+
+// UserActivityTime represents a custom time format
+// used by Gitlab in the user/activities response (YYYY-MM-DD)
+type UserActivityTime struct {
+	time.Time
+}
+
+// User Activity represents an entry in the user/activities response
+// LastActivityAt is deprecated and only available for downward compatibility
+//
+// GitLab API docs:
+// https://docs.gitlab.com/ce/api/users.html#get-user-activities-admin-only
+type UserActivity struct {
+	Username       string            `json:"username"`
+	LastActivityOn *UserActivityTime `json:"last_activity_on"`
+	LastActivityAt *UserActivityTime `json:"last_activity_at"`    // deprecated!
+}
+
+// layout of UserActivityTime for parsing time string
+const uatLayout = "2006-01-02"
+
+// custom unmarshaller for for UserActivityTime
+func (uat *UserActivityTime) UnmarshalJSON(b []byte) (err error) {
+	s := strings.Trim(string(b), "\"")
+	if s == "null" {
+		uat.Time = time.Time{}
+		return
+	}
+	uat.Time, err = time.Parse(uatLayout, s)
+	return
+}
+
+// custom marshaller for UserActivityTime
+func (uat *UserActivityTime) MarshalJSON() ([]byte, error) {
+	if uat.Time.UnixNano() == nilTime {
+		return []byte("null"), nil
+	}
+	return []byte(fmt.Sprintf("\"%s\"", uat.Time.Format(uatLayout))), nil
+}
+
+// helper method to check whether UserActivityTime is set
+var nilTime = (time.Time{}).UnixNano()
+func (uat *UserActivityTime) IsSet() bool {
+	return uat.UnixNano() != nilTime
+}
+
+// Get user activities (admin only)
+//
+// GitLab API docs:
+// https://docs.gitlab.com/ce/api/users.html#get-user-activities-admin-only
+func (s *UsersService) GetUserActivities(options ...OptionFunc) ([]*UserActivity, *Response, error) {
+	req, err := s.client.NewRequest("GET", "user/activities", nil, options)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	var t []*UserActivity
+	resp, err := s.client.Do(req, &t)
+	if err != nil {
+		return nil, resp, err
+	}
+
+	return t, resp, err
+}