Spaced string in imgTag

Hexo v3.0

Using the img tag plugin the Alt attribute gets rendered as part of the the title attribute.

Example code used to test:

```
{% img class-name /path/to/image 100 100 "Title" "Alt" %}
```

HTML Output

``` html
<img src="/path/to/image" class="class-name" width="100" height="100" title="Title Alt">
```
