Be more strict about what constitutes a valid NamespacedKey

* Brings us largely in line with what is supported by the implementation
* Adds unit tests

By: md_5 <git@md-5.net>

paper-api/src/test/java/org/bukkit/NamespacedKeyTest.java

@@ -0,0 +1,49 @@
+package org.bukkit;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class NamespacedKeyTest {
+
+    @Test
+    public void testValid() {
+        Assert.assertEquals("minecraft:foo", new NamespacedKey("minecraft", "foo").toString());
+        Assert.assertEquals("minecraft:foo/bar", new NamespacedKey("minecraft", "foo/bar").toString());
+        Assert.assertEquals("minecraft:foo/bar_baz", new NamespacedKey("minecraft", "foo/bar_baz").toString());
+        Assert.assertEquals("minecraft:foo/bar_baz-qux", new NamespacedKey("minecraft", "foo/bar_baz-qux").toString());
+        Assert.assertEquals("minecraft:foo/bar_baz-qux.quux", new NamespacedKey("minecraft", "foo/bar_baz-qux.quux").toString());
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testEmptyNamespace() {
+        new NamespacedKey("", "foo").toString();
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testEmptyKey() {
+        new NamespacedKey("minecraft", "").toString();
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testInvalidNamespace() {
+        new NamespacedKey("minecraft/test", "foo").toString();
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testInvalidKey() {
+        new NamespacedKey("minecraft", "foo!").toString();
+    }
+
+    @Test
+    public void testBelowLength() {
+        new NamespacedKey("loremipsumdolorsitametconsecteturadipiscingelitduisvolutpatvelitsitametmaximusscelerisquemorbiullamcorperexacconsequategestas",
+                "loremipsumdolorsitametconsecteturadipiscingelitduisvolutpatvelitsitametmaximusscelerisquemorbiullamcorperexacconsequategestas").toString();
+    }
+
+    @Test(expected = IllegalArgumentException.class)
+    public void testAboveLength() {
+        new NamespacedKey("loremipsumdolorsitametconsecteturadipiscingelitduisvolutpatvelitsitametmaximusscelerisquemorbiullamcorperexacconsequategestas",
+                "loremipsumdolorsitametconsecteturadipiscingelitduisvolutpatvelitsitametmaximusscelerisquemorbiullamcorperexacconsequategestas/"
+                + "loremipsumdolorsitametconsecteturadipiscingelitduisvolutpatvelitsitametmaximusscelerisquemorbiullamcorperexacconsequategestas").toString();
+    }
+}