Fix and test for image problem

I noticed that this call:

```
   (to-clj (mp "![alt text](/url \"image title\")"))
```

causes an exception:  ClassCastException clojure.lang.LazySeq cannot be cast to java.lang.CharSequence  clojure.string/blank? (string.clj:231)

The problem seems to be in the way the :alt attribute is handled in ExpImageNode:

```
(extend-type ExpImageNode AstToClj
  (to-clj [node] {:tag :img
                  :attrs (a-attrs
                          {:src (.url node)
                           :title (.title node)
                           :alt (clj-contents node)})}))
```

The clj-contents returns a sequence. Later this passed to clojure.string/blank? which tries to cast it to CharSequence. I assume that the contents of :alt should be a string, so if I change the last line to:

```
                           :alt (str/join (clj-contents node))
```

then:

```
   => (to-clj (mp "![alt text](/url \"image title\")"))
   ({:tag :p, :content ({:tag :img, :attrs {:src "/url", :title "image title", :alt "alt text"}})})
```

which seems to be right.

This pull request contains that fix and a simple test for it.