Refine WebFlux Coroutines API

This commit provides both nullable and non-nullable variants for
awaitBody, makes awaitPrincipal return type nullable and rename
awaitResponse to awaitExchange for better consistency with Java API.

See gh-19975

spring-webflux/src/test/kotlin/org/springframework/web/reactive/function/client/ClientResponseExtensionsTests.kt

@@ -21,6 +21,7 @@ import io.mockk.mockk
 import io.mockk.verify
 import kotlinx.coroutines.runBlocking
 import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNull
 import org.junit.Test
 import org.springframework.core.ParameterizedTypeReference
 import org.springframework.http.HttpStatus
@@ -69,6 +70,15 @@ class ClientResponseExtensionsTests {
 		}
 	}
 
+	@Test
+	fun awaitBodyOrNull() {
+		val response = mockk<ClientResponse>()
+		every { response.bodyToMono<String>() } returns Mono.empty()
+		runBlocking {
+			assertNull(response.awaitBodyOrNull<String>())
+		}
+	}
+
 	@Test
 	fun awaitEntity() {
 		val response = mockk<ClientResponse>()

spring-webflux/src/test/kotlin/org/springframework/web/reactive/function/client/WebClientExtensionsTests.kt

@@ -58,11 +58,11 @@ class WebClientExtensionsTests {
 	}
 
 	@Test
-	fun awaitResponse() {
+	fun awaitExchange() {
 		val response = mockk<ClientResponse>()
 		every { requestBodySpec.exchange() } returns Mono.just(response)
 		runBlocking {
-			assertEquals(response, requestBodySpec.awaitResponse())
+			assertEquals(response, requestBodySpec.awaitExchange())
 		}
 	}
 

spring-webflux/src/test/kotlin/org/springframework/web/reactive/function/server/ServerRequestExtensionsTests.kt

@@ -61,10 +61,10 @@ class ServerRequestExtensionsTests {
 	}
 
 	@Test
-	fun awaitBodyNull() {
+	fun awaitBodyOrNull() {
 		every { request.bodyToMono<String>() } returns Mono.empty()
 		runBlocking {
-			assertNull(request.awaitBody<String>())
+			assertNull(request.awaitBodyOrNull<String>())
 		}
 	}