Add a section to the config file for HTTP headers to add to responses

The example configuration files add X-Content-Type-Options: nosniff.

Add coverage in existing registry/handlers unit tests.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

docs/handlers/api_test.go

@@ -30,6 +30,10 @@ import (
 	"golang.org/x/net/context"
 )
 
+var headerConfig = http.Header{
+	"X-Content-Type-Options": []string{"nosniff"},
+}
+
 // TestCheckAPI hits the base endpoint (/v2/) ensures we return the specified
 // 200 OK response.
 func TestCheckAPI(t *testing.T) {
@@ -215,6 +219,7 @@ func TestURLPrefix(t *testing.T) {
 		},
 	}
 	config.HTTP.Prefix = "/test/"
+	config.HTTP.Headers = headerConfig
 
 	env := newTestEnvWithConfig(t, &config)
 
@@ -1009,6 +1014,8 @@ func newTestEnv(t *testing.T, deleteEnabled bool) *testEnv {
 		},
 	}
 
+	config.HTTP.Headers = headerConfig
+
 	return newTestEnvWithConfig(t, &config)
 }
 
@@ -1225,6 +1232,14 @@ func checkResponse(t *testing.T, msg string, resp *http.Response, expectedStatus
 
 		t.FailNow()
 	}
+
+	// We expect the headers included in the configuration
+	if !reflect.DeepEqual(resp.Header["X-Content-Type-Options"], []string{"nosniff"}) {
+		t.Logf("missing or incorrect header X-Content-Type-Options %s", msg)
+		maybeDumpResponse(t, resp)
+
+		t.FailNow()
+	}
 }
 
 // checkBodyHasErrorCodes ensures the body is an error body and has the