aws/credentials: Fix shared credential providers filename (#1312)
aws/credentials: Fix shared credential providers filename (#1312)

In #1308 the filename was being retrieved with the SharedConfigFilename
utility function not SharedCredentialsFilename function.

This bug caused the session.New with the SharedCredentials file to break.

Fix #1313
calgaryscientific/aws-sdk-go,calgaryscientific/aws-sdk-go,xibz/aws-sdk-go,yxd-hde/aws-sdk-go,calgaryscientific/aws-sdk-go,radeksimko/aws-sdk-go,agworld/aws-sdk-go,radeksimko/aws-sdk-go,xibz/aws-sdk-go,xibz/aws-sdk-go,aws/aws-sdk-go,yxd-hde/aws-sdk-go,yxd-hde/aws-sdk-go,agworld/aws-sdk-go,yxd-hde/aws-sdk-go,radeksimko/aws-sdk-go,jasdel/aws-sdk-go,xibz/aws-sdk-go,agworld/aws-sdk-go,radeksimko/aws-sdk-go,calgaryscientific/aws-sdk-go,agworld/aws-sdk-go