Adds Enum support to YamlConfiguration class (#41)

Core/src/main/java/com/songoda/core/configuration/yaml/YamlConfiguration.java

@@ -117,6 +117,8 @@ public class YamlConfiguration implements IConfiguration, HeaderCommentable, Nod
                 value = ((Float) value).doubleValue();
             } else if (value instanceof Character) {
                 value = ((Character) value).toString();
+            } else if (value.getClass().isEnum()) {
+                value = ((Enum<?>) value).name();
             } else if (value.getClass().isArray()) {
                 if (value instanceof int[]) {
                     value = Arrays.asList(ArrayUtils.toObject((int[]) value));

Core/src/test/java/com/songoda/core/configuration/yaml/YamlConfigurationTest.java

@@ -289,6 +289,16 @@ class YamlConfigurationTest {
         assertEquals(3, primitivesList.get(2));
     }
 
+    @Test
+    void testSetterWithEnumValue() {
+        final YamlConfiguration cfg = new YamlConfiguration();
+
+        assertNull(cfg.set("primitives.enum", TestEnum.ENUM_VALUE));
+
+        assertInstanceOf(String.class, cfg.get("primitives.enum"));
+        assertEquals(TestEnum.ENUM_VALUE, TestEnum.valueOf((String) cfg.get("primitives.enum")));
+    }
+
     @Test
     void testSetterWithBooleanArrayValue() {
         final YamlConfiguration cfg = new YamlConfiguration();
@@ -566,4 +576,13 @@ class YamlConfigurationTest {
 
         assertEquals(expectedYaml, cfg.toYamlString());
     }
+
+    private enum TestEnum {
+        ENUM_VALUE;
+
+        @Override
+        public String toString() {
+            return "#toString(): " + super.toString();
+        }
+    }
 }