[API] Extend ImString to accept more objects to set string from

Also make getBytes method to use UTF-8 charset

imgui-binding/src/test/java/imgui/ImStringTest.java

@@ -32,28 +32,60 @@ class ImStringTest {
     }
 
     @Test
-    public void testSet() {
+    public void testSetWhenEmpty() {
         final ImString str = new ImString(3);
         assertEquals("", str.get());
         assertEquals(0, str.getLength());
         assertEquals(3 + 1, str.getBufferSize());
+    }
 
+    @Test
+    public void testSetWhenNotResized() {
+        final ImString str = new ImString(3);
         str.set("test");
         assertEquals("tes", str.get());
         assertEquals(3, str.getLength());
         assertEquals(3 + 1, str.getBufferSize());
+    }
 
+    @Test
+    public void testSetWhenResizedWithZeroResizeValue() {
+        final ImString str = new ImString(3);
         str.set("test", true, 0);
         assertEquals("test", str.get());
         assertEquals(4, str.getLength());
         assertEquals(4 + 1, str.getBufferSize());
+    }
 
+    @Test
+    public void testSetWhenResized() {
+        final ImString str = new ImString(3);
         str.set("0123456789", true);
         assertEquals("0123456789", str.get());
         assertEquals(10, str.getLength());
         assertEquals(10 + str.inputData.resizeFactor + 1, str.getBufferSize());
+    }
 
-        str.set(null);
+    @Test
+    public void testSetWhenNullObject() {
+        final ImString str = new ImString();
+        final Object object = null;
+        str.set(object);
         assertEquals("null", str.get());
     }
+
+    @Test
+    public void testSetWhenImString() {
+        final ImString str = new ImString();
+        final ImString string = new ImString("test");
+        str.set(string);
+        assertEquals("test", str.get());
+    }
+
+    @Test
+    public void testSetWhenNumber() {
+        final ImString str = new ImString();
+        str.set(123);
+        assertEquals("123", str.get());
+    }
 }