Exception when using an empty "secret key" string

[jamshid]

If you get an error like this, you're creating AWS4Auth() with an empty "secret key" string.

>>> requests_aws4auth.AWS4Auth('username','', 'eu-west-1', 's3')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/site-packages/requests_aws4auth/aws4auth.py", line 246, in __init__
    self.regenerate_signing_key(secret_key=secret_key)
  File "/usr/lib/python2.7/site-packages/requests_aws4auth/aws4auth.py", line 291, in regenerate_signing_key
    secret_key = secret_key or self.signing_key.secret_key
AttributeError: 'NoneType' object has no attribute 'secret_key'

It's not something real users would want to do, but found this in testing code that previously ran with V2 Signature awsauth.S3Auth. This patch works around the error:

diff -u /usr/lib/python2.7/site-packages/requests_aws4auth/aws4auth.py~ /usr/lib/python2.7/site-packages/requests_aws4auth/aws4auth.py
--- /usr/lib/python2.7/site-packages/requests_aws4auth/aws4auth.py~	2017-02-15 19:17:02.000000000 +0000
+++ /usr/lib/python2.7/site-packages/requests_aws4auth/aws4auth.py	2017-02-18 00:18:15.425646504 +0000
@@ -288,7 +288,7 @@
                                    self.signing_key.secret_key is None):
             raise NoSecretKeyError
 
-        secret_key = secret_key or self.signing_key.secret_key
+        secret_key = secret_key or (self.signing_key and self.signing_key.secret_key) or ''
         region = region or self.region
         service = service or self.service
         date = date or self.date

[aalvrz]

@sam-washington I am experiencing this behavior using fallback env variables in my unit tests and it's very annoying. Could we change this behavior?