fix: enable select current element for struct slice unmarshal

[xzvwang]

Once I met a html like this:

<div>
    <a href="url1">Name 1</a>
    <a href="url2">Name 2</a>
</div>

and I want extract the data as an array of name and url pair, so i write a selector:

[]*struct {
    Name string `selector:""`
    URL  string `selector:"" attr:"href"`
} `selector:"a"`

but the Unmarshal can not do it well, so i expand this function, add unmarshalString to handle if selector is empty string.