Encoding error in helper get_responsible_party

Hello,

I am using the helper "get_responsible_party" but it doesn't work with my metadata

An example is when the field "responsible-party" has the value:

```
[{"name": "Instituto Geol\u00f3gico y Minero de Espa\u00f1a (IGME)", "roles": ["pointOfContact"]}]
```

When i watch the detail metadata page, the field is showed in json and in the log this message is showed:

```
'ascii' codec can't encode character u'\xf3' in position 14: ordinal not in range(128)
```

The error is in the line 54 from the file ckanext-spatial/ckanext/spatial/helpers.py:

```
h.json.loads(value)
```