Revise RowMetaData#getColumnMetadata(String)

Motivation:
The current behavior does not align with the specification

Modifications:
implement the intended behavior as per the specification.

Result:
Correct behavior

r2dbc-mysql/src/main/java/io/asyncer/r2dbc/mysql/InsertSyntheticRow.java

@@ -52,15 +52,11 @@ final class InsertSyntheticRow implements MySqlRow, MySqlRowMetadata, MySqlColum
 
     private final long lastInsertId;
 
-    private final ColumnNameSet nameSet;
-
     InsertSyntheticRow(Codecs codecs, String keyName, long lastInsertId) {
         this.codecs = requireNonNull(codecs, "codecs must not be null");
         this.keyName = requireNonNull(keyName, "keyName must not be null");
         // lastInsertId may be negative if key is BIGINT UNSIGNED and value overflow than signed int64.
         this.lastInsertId = lastInsertId;
-        // Singleton name must be sorted.
-        this.nameSet = ColumnNameSet.of(keyName);
     }
 
     @Override
@@ -164,14 +160,14 @@ public <T> T get(String name, ParameterizedType type) {
             lastInsertId < 0 ? Long.toUnsignedString(lastInsertId) : lastInsertId));
     }
 
-    private void assertValidName(String name) {
-        if (!contains0(name)) {
-            throw new NoSuchElementException("Column name '" + name + "' does not exist in " + this.nameSet);
-        }
+    private boolean contains0(final String name) {
+        return keyName.equalsIgnoreCase(name);
     }
 
-    private boolean contains0(String name) {
-        return nameSet.contains(name);
+    private void assertValidName(final String name) {
+        if (!contains0(name)) {
+            throw new NoSuchElementException("Column name '" + name + "' does not exist in {" + name + '}');
+        }
     }
 
     private <T> T get0(Class<?> type) {

r2dbc-mysql/src/main/java/io/asyncer/r2dbc/mysql/MySqlColumnDescriptor.java

@@ -23,6 +23,7 @@
 import io.asyncer.r2dbc.mysql.constant.MySqlType;
 import io.asyncer.r2dbc.mysql.message.server.DefinitionMetadataMessage;
 import io.r2dbc.spi.Nullability;
+import org.jetbrains.annotations.VisibleForTesting;
 
 import static io.asyncer.r2dbc.mysql.internal.util.AssertUtils.require;
 import static io.asyncer.r2dbc.mysql.internal.util.AssertUtils.requireNonNull;
@@ -50,7 +51,8 @@ final class MySqlColumnDescriptor implements MySqlColumnMetadata {
 
     private final int collationId;
 
-    private MySqlColumnDescriptor(int index, short typeId, String name, int definitions,
+    @VisibleForTesting
+    MySqlColumnDescriptor(int index, short typeId, String name, int definitions,
         long size, int decimals, int collationId) {
         require(index >= 0, "index must not be a negative integer");
         require(size >= 0, "size must not be a negative integer");