Add missing required parameeter to ManageIncidentAlerts

When making a call to the REST API[1] endpoint to manage alerts for incidents,
an HTTP header ("From") indicating who made the request is required. As a
result, these methods may not have ever worked properly.

So while we're breaking the API, let's remove the deprecated non-context version
to have fewer breaking changes in v2.

Closes #338

[1] https://developer.pagerduty.com/api-reference/b3A6Mjc0ODE0NA-manage-alerts

incident.go

@@ -632,15 +632,12 @@ func (c *Client) GetIncidentAlertWithContext(ctx context.Context, incidentID, al
 }
 
 // ManageIncidentAlerts allows you to manage the alerts of an incident.
-//
-// Deprecated: Use ManageIncidentAlertsWithContext instead.
-func (c *Client) ManageIncidentAlerts(incidentID string, alerts *IncidentAlertList) (*ListAlertsResponse, error) {
-	return c.ManageIncidentAlertsWithContext(context.Background(), incidentID, alerts)
-}
+func (c *Client) ManageIncidentAlerts(ctx context.Context, incidentID, from string, alerts *IncidentAlertList) (*ListAlertsResponse, error) {
+	h := map[string]string{
+		"From": from,
+	}
 
-// ManageIncidentAlertsWithContext allows you to manage the alerts of an incident.
-func (c *Client) ManageIncidentAlertsWithContext(ctx context.Context, incidentID string, alerts *IncidentAlertList) (*ListAlertsResponse, error) {
-	resp, err := c.put(ctx, "/incidents/"+incidentID+"/alerts/", alerts, nil)
+	resp, err := c.put(ctx, "/incidents/"+incidentID+"/alerts/", alerts, h)
 	if err != nil {
 		return nil, err
 	}

incident_test.go

@@ -1,6 +1,7 @@
 package pagerduty
 
 import (
+	"context"
 	"encoding/json"
 	"io/ioutil"
 	"net/http"
@@ -708,12 +709,17 @@ func TestIncident_GetAlert(t *testing.T) {
 	testEqual(t, want, res)
 }
 
-func TestIncident_ManageAlerts(t *testing.T) {
+func TestIncident_ManageIncidentAlerts(t *testing.T) {
 	setup()
 	defer teardown()
 
+	from := "pagerduty@example.com"
+
 	mux.HandleFunc("/incidents/1/alerts/", func(w http.ResponseWriter, r *http.Request) {
 		testMethod(t, r, "PUT")
+		if hdr := r.Header.Get("From"); hdr != "pagerduty@example.com" {
+			t.Errorf("From header = %q, want %q", hdr, from)
+		}
 		_, _ = w.Write([]byte(`{"alerts": [{"id": "1"}]}`))
 	})
 
@@ -730,7 +736,7 @@ func TestIncident_ManageAlerts(t *testing.T) {
 			},
 		},
 	}
-	res, err := client.ManageIncidentAlerts(incidentID, input)
+	res, err := client.ManageIncidentAlerts(context.Background(), incidentID, from, input)
 
 	want := &ListAlertsResponse{
 		Alerts: []IncidentAlert{