Problem:

Holder-of-Key assertions are used to achieve higher levels of federation security, compared to bearer assertions, by having Relying Party challenge subscriber to prove possession of the key specified in the assertion that represents subscriber in addition to verifying the assertion itself signed by Identity Provider. More information about it can be found in https://pages.nist.gov/800-63-3/sp800-63c.html

This library fails to parase SAML respones containing assertions with Holder-of-Key profile, for example:
```
<ns1:SubjectConfirmation Method="urn:oasis:names:tc:SAML:2.0:cm:holder-of-key">
    <ns1:SubjectConfirmationData InResponseTo="id-KHlas49TtW2VdC8WN" NotOnOrAfter="2019-05-14T20:36:13Z" Recipient="https://sp:443/.auth/saml/login">
        <ns2:KeyInfo>
            <ns2:X509Data>
                <ns2:X509Certificate>MIICITCCAYoCAQEwDQYJKoZIhvcNAQELBQAwWDELMAkGA1UEBhMCenoxCzAJBgNVBAgMAnp6MQ0wCwYDVQQHDAR6enp6MQ4wDAYDVQQKDAVaenp6ejEOMAwGA1UECwwFWnp6enoxDTALBgNVBAMMBHRlc3QwIBcNMTkwNDEyMTk1MDM0WhgPMzAxODA4MTMxOTUwMzRaMFgxCzAJBgNVBAYTAnp6MQswCQYDVQQIDAJ6ejENMAsGA1UEBwwEenp6ejEOMAwGA1UECgwFWnp6enoxDjAMBgNVBAsMBVp6enp6MQ0wCwYDVQQDDAR0ZXN0MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDHcj80WU/XBsd9FlyQmfjPUdfmedhCFDd6TEQmZNNqP/UG+VkGa+BXjRIHMfic/WxPTbGhCjv68ci0UDNomUXagFexLGNpkwa7+CRVtoc/1xgq+ySE6M4nhcCutScoxNvWNn5eSQ66i3U0sTv91MgsXxqEdTaiZg0BIufEc3dueQIDAQABMA0GCSqGSIb3DQEBCwUAA4GBAGUV5B+USHvaRa8kgCNJSuNpo6ARlv0ekrk8bbdNRBiEUdCMyoGJFfuM9K0zybX6Vr25wai3nvaog294Vx/jWjX2g5SDbjItH6VGy6C9GCGf1A07VxFRCfJn5tA9HuJjPKiE+g/BmrV5N4CealzFxPHWYkNOzoRU8qI7OqUai1kL</ns2:X509Certificate>
            </ns2:X509Data>
        </ns2:KeyInfo>
    </ns1:SubjectConfirmationData>
</ns1:SubjectConfirmation>
```

fails to be parsed with the following error:
```
ERROR    saml2.response:response.py:836 get subject
Traceback (most recent call last):
  File "/home/abublich/repos/abliqo-pysaml2/venv/local/lib/python2.7/site-packages/pysaml2-4.7.0-py2.7.egg/saml2/response.py", line 828, in _assertion
    self.get_subject()
  File "/home/abublich/repos/abliqo-pysaml2/venv/local/lib/python2.7/site-packages/pysaml2-4.7.0-py2.7.egg/saml2/response.py", line 753, in get_subject
    if not self._holder_of_key_confirmed(_data):
  File "/home/abublich/repos/abliqo-pysaml2/venv/local/lib/python2.7/site-packages/pysaml2-4.7.0-py2.7.egg/saml2/response.py", line 730, in _holder_of_key_confirmed
    [samlp, saml, xenc, ds]):
  File "/home/abublich/repos/abliqo-pysaml2/venv/local/lib/python2.7/site-packages/pysaml2-4.7.0-py2.7.egg/saml2/__init__.py", line 1004, in extension_elements_to_elements
    for extension_element in extension_elements:
TypeError: 'SubjectConfirmationData' object is not iterable
```

The root cause is two-fold:
1. The type SubjectConfirmationDataType_ does not declare KeyInfo as child element.
2. The bug in function _holder_of_key_confirmed: it should check KeyInfo child element of SubjectConfirmationData instead of SubjectConfirmationData itself.

Solution:
Fixed the root cause and added new unit tests that verify successful parsing of Holder-of-Key assertions.