use PUT verb when setting org membership

Fixes #244

github/orgs_members.go

@@ -238,14 +238,16 @@ func (s *OrganizationsService) GetOrgMembership(user, org string) (*Membership,
 // GitHub API docs: https://developer.github.com/v3/orgs/members/#add-or-update-organization-membership
 // GitHub API docs: https://developer.github.com/v3/orgs/members/#edit-your-organization-membership
 func (s *OrganizationsService) EditOrgMembership(user, org string, membership *Membership) (*Membership, *Response, error) {
-	var u string
+	var u, method string
 	if user != "" {
 		u = fmt.Sprintf("orgs/%v/memberships/%v", org, user)
+		method = "PUT"
 	} else {
 		u = fmt.Sprintf("user/memberships/orgs/%v", org)
+		method = "PATCH"
 	}
 
-	req, err := s.client.NewRequest("PATCH", u, membership)
+	req, err := s.client.NewRequest(method, u, membership)
 	if err != nil {
 		return nil, nil, err
 	}

github/orgs_members_test.go

@@ -321,7 +321,7 @@ func TestOrganizationsService_EditOrgMembership_SpecifiedUser(t *testing.T) {
 		v := new(Membership)
 		json.NewDecoder(r.Body).Decode(v)
 
-		testMethod(t, r, "PATCH")
+		testMethod(t, r, "PUT")
 		if !reflect.DeepEqual(v, input) {
 			t.Errorf("Request body = %+v, want %+v", v, input)
 		}