Derive mode from DSN function

- Add headers in all files
- Minor field name changes

src/FieldMatch.php

@@ -1,4 +1,10 @@
-<?php declare(strict_types=1);
+<?php
+/**
+ * @author Daniel J. Summers <daniel@bitbadger.solutions>
+ * @license MIT
+ */
+
+declare(strict_types=1);
 
 namespace BitBadger\PDODocument;
 
@@ -18,7 +24,7 @@ enum FieldMatch
      *
      * @return string The SQL keyword for this enumeration value
      */
-    public function toString(): string
+    public function toSQL(): string
     {
         return match ($this) {
             FieldMatch::All => 'AND',