Template property with unspecified value should have a better behavior

[LastLeaf]

Currently, when a property value is unspecified (e.g. <div some-prop> ) in WXML, it will be treated as {{true}} (e.g. <div some-prop="{{ true }}"> ).

This behavior works on most pratical cases, since this usually happens on boolean properties. However, if it is used in non-boolean properties, this behavior is strange.