Fix set trace attributes for django when host is not allowed

`request.get_host()` raises DisallowedHost exception if request came with HOST witch not configured
in ALLOWED_HOSTS setting.

Django provides special methods `request._get_raw_host()` and `request.get_raw_uri()` to get values
like `get_host()` and `build_absolute_uri()` but without checking by ALLOWED_HOSTS.

Without fix error will be like:

```
ERROR    opencensus.ext.django.middleware:middleware.py:235 Failed to trace request
Traceback (most recent call last):
  File "/home/konst/develop/opencensus-python/contrib/opencensus-ext-django/opencensus/ext/django/middleware.py", line 211, in process_request
    attribute_value=request.get_host())
  File "/home/konst/develop/opencensus-python/.nox/unit-2-7/lib/python2.7/site-packages/django/http/request.py", line 113, in get_host
    raise DisallowedHost(msg)
DisallowedHost: Invalid HTTP_HOST header: 'notallowedhost'. You may need to add u'notallowedhost' to ALLOWED_HOSTS.
```

contrib/opencensus-ext-django/opencensus/ext/django/middleware.py

@@ -208,7 +208,7 @@ def process_request(self, request):
             span.span_kind = span_module.SpanKind.SERVER
             tracer.add_attribute_to_current_span(
                 attribute_key=HTTP_HOST,
-                attribute_value=request.get_host())
+                attribute_value=request._get_raw_host())
             tracer.add_attribute_to_current_span(
                 attribute_key=HTTP_METHOD,
                 attribute_value=request.method)
@@ -220,7 +220,7 @@ def process_request(self, request):
                 attribute_value=str(request.path))
             tracer.add_attribute_to_current_span(
                 attribute_key=HTTP_URL,
-                attribute_value=str(request.build_absolute_uri()))
+                attribute_value=str(request.get_raw_uri()))
 
             # Add the span to thread local
             # in some cases (exceptions, timeouts) currentspan in

contrib/opencensus-ext-django/tests/test_django_middleware.py

@@ -17,7 +17,7 @@
 import unittest
 
 import mock
-from django.test import RequestFactory
+from django.test import RequestFactory, override_settings
 from django.test.utils import teardown_test_environment
 
 from opencensus.trace import execution_context, print_exporter, samplers
@@ -129,6 +129,59 @@ def test_process_request(self):
 
         self.assertEqual(span.name, 'mock.mock.Mock')
 
+    @override_settings(ALLOWED_HOSTS=['allowedhost'])
+    def test_process_request_with_disallowed_host(self):
+        from opencensus.ext.django import middleware
+
+        trace_id = '2dd43a1d6b2549c6bc2a1a54c2fc0b05'
+        span_id = '6e0c63257de34c92'
+        django_trace_id = '00-{}-{}-00'.format(trace_id, span_id)
+
+        django_request = RequestFactory().get('/wiki/Rabbit', **{
+            'HTTP_HOST': 'notallowedhost',
+            'HTTP_TRACEPARENT': django_trace_id})
+
+        # Force the test request to be sampled
+        settings = type('Test', (object,), {})
+        settings.OPENCENSUS = {
+            'TRACE': {
+                'SAMPLER': 'opencensus.trace.samplers.AlwaysOnSampler()',  # noqa
+            }
+        }
+        patch_settings = mock.patch(
+            'django.conf.settings',
+            settings)
+
+        with patch_settings:
+            middleware_obj = middleware.OpencensusMiddleware()
+
+        # test process_request
+        middleware_obj.process_request(django_request)
+
+        tracer = middleware._get_current_tracer()
+
+        span = tracer.current_span()
+
+        expected_attributes = {
+            'http.host': u'notallowedhost',
+            'http.method': 'GET',
+            'http.path': u'/wiki/Rabbit',
+            'http.route': u'/wiki/Rabbit',
+            'http.url': u'http://notallowedhost/wiki/Rabbit',
+        }
+        self.assertEqual(span.span_kind, span_module.SpanKind.SERVER)
+        self.assertEqual(span.attributes, expected_attributes)
+        self.assertEqual(span.parent_span.span_id, span_id)
+
+        span_context = tracer.span_context
+        self.assertEqual(span_context.trace_id, trace_id)
+
+        # test process_view
+        view_func = mock.Mock()
+        middleware_obj.process_view(django_request, view_func)
+
+        self.assertEqual(span.name, 'mock.mock.Mock')
+
     def test_excludelist_path(self):
         from opencensus.ext.django import middleware