Introduce TestRestTemplate Kotlin extensions

This commit introduces Kotlin extensions similar to the RestOperations
ones in order to be able to take advantage of Kotlin reified type
parameters for example.

See gh-11039

spring-boot-project/spring-boot-parent/pom.xml

@@ -53,6 +53,25 @@
 				<artifactId>log4j</artifactId>
 				<version>1.2.17</version>
 			</dependency>
+			<dependency>
+				<groupId>com.nhaarman</groupId>
+				<artifactId>mockito-kotlin</artifactId>
+				<version>1.5.0</version>
+				<exclusions>
+					<exclusion>
+						<groupId>org.jetbrains.kotlin</groupId>
+						<artifactId>kotlin-stdlib</artifactId>
+					</exclusion>
+					<exclusion>
+						<groupId>org.jetbrains.kotlin</groupId>
+						<artifactId>kotlin-reflect</artifactId>
+					</exclusion>
+					<exclusion>
+						<groupId>org.mockito</groupId>
+						<artifactId>mockito-core</artifactId>
+					</exclusion>
+				</exclusions>
+			</dependency>
 			<dependency>
 				<groupId>com.squareup.okhttp</groupId>
 				<artifactId>okhttp</artifactId>