Allow copy-constructors applied to Collections

Instead of requiring a List, which is unnecessary strict. This aligns
with the JDK copy constructors for lists:

public ArrayList(Collection<? extends E> c)
https://docs.oracle.com/javase/8/docs/api/java/util/ArrayList.html#ArrayList-java.util.Collection-

List.copyOf
https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/List.html#copyOf(java.util.Collection)

src/main/java/no/digipost/collection/NonEmptyList.java

@@ -18,6 +18,7 @@
 import no.digipost.stream.NonEmptyStream;
 
 import java.util.ArrayList;
+import java.util.Collection;
 import java.util.List;
 import java.util.Optional;
 
@@ -189,8 +190,8 @@ static <E> NonEmptyList<E> ofUnsafe(E[] nonEmptyArray) {
      * @return the resulting non-empty list,
      *         or {@link Optional#empty()} if the given list is empty
      */
-    static <E> Optional<NonEmptyList<E>> copyOf(List<E> list) {
-        return firstOf(list).map(first -> NonEmptyList.of(first, new ArrayList<>(list.subList(1, list.size()))));
+    static <E> Optional<NonEmptyList<E>> copyOf(Collection<E> collection) {
+        return of(new ArrayList<>(collection));
     }
 
 
@@ -227,7 +228,7 @@ static <E> Optional<NonEmptyList<E>> copyOf(E[] array) {
      *
      * @see #copyOf(List)
      */
-    static <E> NonEmptyList<E> copyOfUnsafe(List<E> nonEmptyList) {
+    static <E> NonEmptyList<E> copyOfUnsafe(Collection<E> nonEmptyList) {
         return copyOf(nonEmptyList).orElseThrow(() -> new IllegalArgumentException("empty list"));
     }