Make ref_resolver.Loader more fault tolerant

Currently it falls over is HOME environment variable is not set.

Author: tjelvar-olsson

schema_salad/ref_resolver.py

@@ -204,7 +204,7 @@ def __init__(self,
         else:
             self.cache = {}
 
-        if session is None:
+        if (session is None) and ("HOME" in os.environ):
             self.session = CacheControl(requests.Session(),
                                    cache=FileCache(os.path.join(os.environ["HOME"], ".cache", "salad")))
         else:

schema_salad/tests/test_ref_resolver.py

@@ -0,0 +1,10 @@
+"""Test the ref_resolver module."""
+
+def test_Loader_initialisation_when_HOME_env_is_missing():
+    from schema_salad.ref_resolver import Loader
+    import os
+
+    # Simulate missing HOME environment variable.
+    if "HOME" in os.environ:
+        del os.environ["HOME"]
+    Loader(ctx={})