Use canonical way to check if a map contains a key

As the language spec:
https://golang.org/ref/spec#Index_expressions

Signed-off-by: Hu Keping <hukeping@huawei.com>

signer/api/api.go

@@ -34,14 +34,12 @@ func getCryptoService(w http.ResponseWriter, algorithm string, cryptoServices si
 		return nil
 	}
 
-	service := cryptoServices[algorithm]
-
-	if service == nil {
-		http.Error(w, "algorithm "+algorithm+" not supported", http.StatusBadRequest)
-		return nil
+	if service, ok := cryptoServices[algorithm]; ok {
+		return service
 	}
 
-	return service
+	http.Error(w, "algorithm "+algorithm+" not supported", http.StatusBadRequest)
+	return nil
 }
 
 // KeyInfo returns a Handler that given a specific Key ID param, returns the public key bits of that key

signer/api/api_test.go

@@ -225,3 +225,26 @@ func TestSignHandlerReturns404WithNonexistentKey(t *testing.T) {
 
 	assert.Equal(t, 404, res.StatusCode)
 }
+
+func TestCreateKeyHandlerWithInvalidAlgorithm(t *testing.T) {
+	keyStore := trustmanager.NewKeyMemoryStore(passphraseRetriever)
+	cryptoService := cryptoservice.NewCryptoService("", keyStore)
+	setup(signer.CryptoServiceIndex{data.ED25519Key: cryptoService, data.RSAKey: cryptoService, data.ECDSAKey: cryptoService})
+
+	// The `rbtree-algorithm` is expected as not supported
+	createKeyURL := fmt.Sprintf("%s/%s", createKeyBaseURL, "rbtree-algorithm")
+
+	request, err := http.NewRequest("POST", createKeyURL, nil)
+	assert.Nil(t, err)
+
+	res, err := http.DefaultClient.Do(request)
+	assert.Nil(t, err)
+
+	assert.Equal(t, http.StatusBadRequest, res.StatusCode)
+
+	body, err := ioutil.ReadAll(res.Body)
+	assert.Nil(t, err)
+
+	// The body may contains some `\r\n`, so we use assert.Contains not assert.Equals
+	assert.Contains(t, string(body), "algorithm rbtree-algorithm not supported")
+}