VocabString class lookup failures

[bworrell]

If I try to parse an input document with a custom CV without registering a VocabString class for it, I get a parse error. This is on python-stix 1.2.0.1.dev0.

My foobar:FooVocab-1.0 instance below causes problems, but shouldn't.

<stix:STIX_Package 
    xmlns:coa="http://stix.mitre.org/CourseOfAction-1"
    xmlns:cybox="http://cybox.mitre.org/cybox-2"
    xmlns:cyboxCommon="http://cybox.mitre.org/common-2"
    xmlns:cyboxVocabs="http://cybox.mitre.org/default_vocabularies-2"
    xmlns:example="http://example.com"
    xmlns:incident="http://stix.mitre.org/Incident-1"
    xmlns:stix="http://stix.mitre.org/stix-1"
    xmlns:stixCommon="http://stix.mitre.org/common-1"
    xmlns:stixVocabs="http://stix.mitre.org/default_vocabularies-1"
    xmlns:foobar="a:testing:namespace"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" id="example:Package-73ce966d-52d2-4092-af41-114e45721814" version="1.2" timestamp="2015-06-02T20:21:54.139127+00:00">
    <stix:Incidents>
        <stix:Incident id="example:incident-b44bc002-4f4c-4dea-ab8b-2dbef815d016" timestamp="2015-06-02T20:21:54.139254+00:00" xsi:type='incident:IncidentType'>
            <incident:Title>Breach</incident:Title>
            <incident:COA_Requested>
                <incident:Course_Of_Action id="example:coa-9b5c8e6f-c7e4-45dc-812e-098d455bf023" timestamp="2015-06-02T20:21:54.139444+00:00" xsi:type='coa:CourseOfActionType'>
                    <coa:Title>Monitor activity related to known compromised accounts</coa:Title>
                    <coa:Type xsi:type="foobar:FooVocab-1.0">THIS SHOULD BE FINE</coa:Type>
                    <coa:Objective>
                        <coa:Description>This will further our investigation into the intruders who are re-using compromised accounts.</coa:Description>
                    </coa:Objective>
                </incident:Course_Of_Action>
            </incident:COA_Requested>
        </stix:Incident>
    </stix:Incidents>
</stix:STIX_Package>

Parse output:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "stix/core/stix_package.py", line 508, in from_xml
    return entity_parser.parse_xml(xml_file, encoding=encoding)
  File "stix/utils/parser.py", line 259, in parse_xml
    stix_package = STIXPackage().from_obj(stix_package_obj)
  File "stix/core/stix_package.py", line 455, in from_obj
    return_obj.incidents = Incidents.from_obj(obj.Incidents)
  File "stix/base.py", line 384, in from_obj
    return_obj.append(contained_type.from_obj(item))
  File "stix/incident/__init__.py", line 530, in from_obj
    return_obj.coa_requested = _COAsRequested.from_obj(obj.COA_Requested)
  File "stix/base.py", line 497, in from_obj
    return cls(items)
  File "stix/base.py", line 530, in __init__
    TypedCollection.__init__(self, *args)
  File "stix/base.py", line 440, in __init__
    self._initialize_inner(*args)
  File "stix/base.py", line 539, in _initialize_inner
    self.extend(arg)
  File "/home/bworrell/.virtualenvs/python-stix/lib/python2.7/_abcoll.py", line 669, in extend
    for v in values:
  File "stix/base.py", line 496, in <genexpr>
    items = (contained_type.from_obj(x) for x in obj_list)
  File "stix/incident/coa.py", line 124, in from_obj
    super(COARequested, cls).from_obj(obj, return_obj=return_obj)
  File "stix/incident/coa.py", line 61, in from_obj
    return_obj.course_of_action = CourseOfAction.from_obj(obj.Course_Of_Action)
  File "stix/coa/__init__.py", line 209, in from_obj
    return_obj.type_ = VocabString.from_obj(obj.Type)
  File "stix/common/vocabs.py", line 108, in from_obj
    klass = stix.lookup_extension(vocab_obj.xsi_type, default=cls)
  File "stix/__init__.py", line 79, in lookup_extension
    return _lookup_extension(typeinfo)
  File "stix/__init__.py", line 51, in _lookup_extension
    raise ValueError("Unregistered xsi:type %s" % xsi_type)
ValueError: Unregistered xsi:type foobar:FooVocab-1.0