Fix #1712 (issue when django-guardian is installed but not configured/used)

rest_framework/compat.py

@@ -48,12 +48,15 @@ try:
 except ImportError:
     django_filters = None
 
-# guardian is optional
-try:
-    import guardian
-    import guardian.shortcuts  # Fixes #1624
-except ImportError:
-    guardian = None
+# Django-guardian is optional. Import only if guardian is in INSTALLED_APPS
+# Fixes (#1712). We keep the try/except for the test suite.
+guardian = None
+if 'guardian' in settings.INSTALLED_APPS:
+    try:
+        import guardian
+        import guardian.shortcuts  # Fixes #1624
+    except ImportError:
+        pass
 
 
 # cStringIO only if it's available, otherwise StringIO