Improve doc of custom codec

- Correct parameter interface from `Parameter` to `MySqlParameter`
- Override `toString()` in the implementation of `MySqlParameter`

README.md

@@ -454,7 +454,7 @@ public final class JacksonCodec implements Codec<Object> {
         return encoding;
     }
 
-    private static final class JacksonParameter implements Parameter {
+    private static final class JacksonParameter implements MySqlParameter {
 
         private final ByteBufAllocator allocator;
 
@@ -523,6 +523,14 @@ public final class JacksonCodec implements Codec<Object> {
         public short getType() {
             return DataTypes.VARCHAR;
         }
+
+        /**
+         * Optional, for statements/parameters logging.
+         */
+        @Override
+        public String toString() {
+            return value.toString();
+        }
     }
 }
 ```

src/test/java/io/asyncer/r2dbc/mysql/codec/CodecTestSupport.java

@@ -103,7 +103,7 @@ default void encodeStringify() {
 
     @Test
     default void encodedToString() {
-        Codec<T> codec = getCodec(UnpooledByteBufAllocator.DEFAULT);
+        Codec<T> codec = getCodec();
         T[] origin = originParameters();
 
         for (T t : origin) {