Issue #263: test test_read_encrypted_file fails when $GNUPGHOME is not set (blais/beancount)

[Martin Michlmayr]

New issue 263: test test_read_encrypted_file fails when $GNUPGHOME is not set
https://bitbucket.org/blais/beancount/issues/263/test-test_read_encrypted_file-fails-when

Martin Michlmayr:

I get:

```
ERROR: test_read_encrypted_file (beancount.utils.encryption_test.TestEncryptedFiles)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/home/tbm/beancount/beancount/utils/encryption_test.py", line 132, in test_read_encrypted_file
self.assertEqual(INPUT, plaintext)
File "/usr/lib/python3.6/contextlib.py", line 88, in __exit__
next(self.gen)
File "/home/tbm/beancount/beancount/utils/test_utils.py", line 359, in environ
os.environ[varname] = oldvalue
File "/usr/lib/python3.6/os.py", line 674, in __setitem__
value = self.encodevalue(value)
File "/usr/lib/python3.6/os.py", line 744, in encode
raise TypeError("str expected, not %s" % type(value).__name__)
TypeError: str expected, not NoneType
```

I don't have `$GNUPGHOME` defined and it barfs when it tries to set the old value after the test (since there's no old value).

I'll propose a PR.