Replace casts to ByteBuffer after ByteBuffer.flip()

Prepare code compatibility for newer Java versions that introduced overrides for ByteBuffer.flip() returning ByteBuffer instead of Buffer.

[closes #340]

Author: mp911de

src/main/java/io/r2dbc/postgresql/message/frontend/GSSResponse.java

@@ -46,8 +46,8 @@ public final class GSSResponse implements FrontendMessage {
      */
     public GSSResponse(ByteBuffer data) {
         Assert.requireNonNull(data, "data must not be null");
-
-        this.data = (ByteBuffer) data.flip();
+        data.flip();
+        this.data = data;
     }
 
     @Override

src/main/java/io/r2dbc/postgresql/message/frontend/SASLInitialResponse.java

@@ -50,7 +50,12 @@ public final class SASLInitialResponse implements FrontendMessage {
      * @throws IllegalArgumentException if {@code name} is {@code null}
      */
     public SASLInitialResponse(@Nullable ByteBuffer initialResponse, String name) {
-        this.initialResponse = initialResponse == null ? null : (ByteBuffer) initialResponse.flip();
+        if (initialResponse == null) {
+            this.initialResponse = null;
+        } else {
+            initialResponse.flip();
+            this.initialResponse = initialResponse;
+        }
         this.name = Assert.requireNonNull(name, "name must not be null");
     }
 

src/main/java/io/r2dbc/postgresql/message/frontend/SASLResponse.java

@@ -46,8 +46,8 @@ public final class SASLResponse implements FrontendMessage {
      */
     public SASLResponse(ByteBuffer data) {
         Assert.requireNonNull(data, "data must not be null");
-
-        this.data = (ByteBuffer) data.flip();
+        data.flip();
+        this.data = data;
     }
 
     @Override