Do not prepend slash to URIs with a scheme.

See gh-206.

spring-vault-core/src/main/java/org/springframework/vault/client/VaultClients.java

@@ -184,6 +184,10 @@ public class VaultClients {
 		 */
 		private String prepareUriTemplate(String uriTemplate) {
 
+			if (uriTemplate.startsWith("http:") || uriTemplate.startsWith("https:")) {
+				return uriTemplate;
+			}
+
 			if (getBaseUrl() != null) {
 				if (uriTemplate.startsWith("/") && getBaseUrl().endsWith("/")) {
 					return uriTemplate.substring(1);

spring-vault-core/src/test/java/org/springframework/vault/client/VaultClientsUnitTests.java

@@ -0,0 +1,68 @@
+/*
+ * Copyright 2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.vault.client;
+
+import java.net.URI;
+
+import org.junit.Test;
+
+import org.springframework.vault.client.VaultClients.PrefixAwareUriTemplateHandler;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Unit tests for
+ * {@link org.springframework.vault.client.VaultClients.PrefixAwareUriTemplateHandler}.
+ *
+ * @author Mark Paluch
+ */
+public class VaultClientsUnitTests {
+
+	@Test
+	public void shouldPrefixRelativeUrl() {
+
+		final VaultEndpoint localhost = VaultEndpoint.create("localhost", 8200);
+		PrefixAwareUriTemplateHandler handler = new PrefixAwareUriTemplateHandler(
+				new VaultEndpointProvider() {
+					@Override
+					public VaultEndpoint getVaultEndpoint() {
+						return localhost;
+					}
+				});
+
+		URI uri = handler.expand("/path/{bar}", "bar");
+
+		assertThat(uri).hasHost("localhost").hasPort(8200).hasPath("/v1/path/bar");
+	}
+
+	@Test
+	public void shouldNotPrefixAbsoluteUrl() {
+
+		final VaultEndpoint localhost = VaultEndpoint.create("localhost", 8200);
+		PrefixAwareUriTemplateHandler handler = new PrefixAwareUriTemplateHandler(
+				new VaultEndpointProvider() {
+					@Override
+					public VaultEndpoint getVaultEndpoint() {
+						return localhost;
+					}
+				});
+
+		URI uri = handler.expand("https://foo/path/{bar}", "bar");
+
+		assertThat(uri).hasScheme("https").hasHost("foo").hasPort(-1)
+				.hasPath("/path/bar");
+	}
+}