Fix for using harvesters with organization setup

With organization setup the harvesting process raises a ValidationError exception

```
2013-02-13 11:18:21,670 DEBUG [ckanext.patstatweb.harvesters] In PatStatWebHarvester fetch_stage
2013-02-13 11:18:21,981 DEBUG [ckanext.patstatweb.harvesters] In PatStatWebHarvester import_stage
2013-02-13 11:18:22,357 INFO  [ckanext.harvest.harvesters.base] Package with GUID 9d15e19d1b17ac432dd704542a384304f7a37992 does not exist, let's create it
2013-02-13 11:18:22,384 ERROR [ckanext.harvest.harvesters.base] {'  junk': 'The input field __junk was not expected.'}
Traceback (most recent call last):
  File "/home/ckan/pyenv/src/ckanext-harvest/ckanext/harvest/harvesters/base.py", line 165, in _create_or_update_package
    new_package = get_action('package_create_rest')(context, package_dict)
  File "/home/ckan/pyenv/src/ckan/ckan/logic/action/create.py", line 723, in     package_create_rest
    dictized_after = _get_action('package_create')(context, dictized_package)
  File "/home/ckan/pyenv/src/ckan/ckan/logic/action/create.py", line 137, in package_create
    raise ValidationError(errors)
ValidationError: {'  junk': 'The input field __junk was not expected.'}
2013-02-13 11:18:22,392 ERROR [ckanext.harvest.harvesters.base] Invalid package with GUID 9d15e19d1b17ac432dd704542a384304f7a37992: {'__junk': ['The input field __junk was not expected.']}
```

This was caused by the absence of ```form_to_db_schema_options``` so it fallbacks to ```form_to_db_schema``` that doesn't take care of the context (that was patched by the harvester plugin, see https://github.com/okfn/ckanext-harvest/blob/master/ckanext/harvest/harvesters/base.py#L117)