Fix wrong treatment of fields in case of unicode strings.

This patch fixes error when xml_extractor parses an attribute with unicode.

Example:
```
<page>
  <title foobar="漢字" />
</page>
```

Error log:
```
2013-07-11 11:34:02.976 [error] <0.1275.0>@yz_kv:index:168 failed to index object {<<"wikipedia">>,<<"0_3">>} with error badarg because [{erlang,list_to_binary,[[28450,23383]],[]},{yz_solr,encode_field,1,[{file,"src/yz_solr.erl"},{line,304}]},{lists,map,2,[{file,"lists.erl"},{line,1173}]},{yz_solr,encode_doc,1,[{file,"src/yz_solr.erl"},{line,301}]},{yz_solr,'-index/3-lc$^0/1-0-',1,[{file,"src/yz_solr.erl"},{line,156}]},{yz_solr,index,3,[{file,"src/yz_solr.erl"},{line,156}]},{yz_kv,index,8,[{file,"src/yz_kv.erl"},{line,163}]},{riak_kv_vnode,perform_put,3,[{file,"src/riak_kv_vnode.erl"},{line,906}]}]
```