Add some simple compile tests and fix to use a double quoted template value in a tag attribute

Hi,

I wrote some codes to allow to use a double quoted template value in a double quoted tag attribute. Here is the example

```
div class="dialog {{ if eq .Attr "important" }}color-red text-big{{end}}" text
```

Without my fix, it doesn't work. Of course, instead of it,

```
div class="dialog {{ if eq .Attr `important` }}color-red text-big{{end}}" text
```

works but I think it is straightforward allowing both a double quote and a backtick quote in an attribute.

Before fixing this, not to break current behavior, I added simple compile tests to check whether the compiler returns expected values. Those mightn't be perfect so please be careful to merge this.
@tatsushid  BTW, I didn't know that I could get the template's string by doing this. How did you know this way? By reading source code?