restoring default credentials object

[jfwarner]

Fix for #84

Restoring default credentials object to "a chain of credential providers to search for credentials in environment variables, shared credential file, and EC2 Instance Roles"

http://docs.aws.amazon.com/sdk-for-go/api/aws/Config.html#Credentials-field

Previously, the "Credentials" field was left nil if there was no "--profile" argument. Now the Credentials field will be left at the default.

[kahing]

I was always under the assumption that the default value for a struct pointer is nil, is that not the case?

[jfwarner]

I am a Go novice, so I don't quite understand everything that is going on yet. But this documentation leads me to believe otherwise:
http://docs.aws.amazon.com/sdk-for-go/api/aws/Config.html#Credentials-field

I also tested this change, and it behaves exactly the way the above documentation describes.

[jfwarner]

Perhaps it is the case that a nil value induces the desired behavior in the SDK, as in:
if( field is nil) then { use default chain };

In either case, it appears to work.

[kahing]

I found this difficult to believe because if --profile is not used, awsConfig.Credentials is nil before and after the patch.

I will accept this anyway because it makes the config handling more consistent with other overrides.