fix(runtime-core): should preserve props casing when component has no declared props

close #583

packages/runtime-core/__tests__/rendererAttrsFallthrough.spec.ts

@@ -32,7 +32,8 @@ describe('attribute fallthrough', () => {
             id: 'test',
             class: 'c' + count.value,
             style: { color: count.value ? 'red' : 'green' },
-            onClick: inc
+            onClick: inc,
+            'data-id': 1
           })
       }
     }
@@ -66,6 +67,7 @@ describe('attribute fallthrough', () => {
     expect(node.getAttribute('class')).toBe('c2 c0')
     expect(node.style.color).toBe('green')
     expect(node.style.fontWeight).toBe('bold')
+    expect(node.dataset.id).toBe('1')
     node.dispatchEvent(new CustomEvent('click'))
     expect(click).toHaveBeenCalled()