aws/session: Fix env vars being correctly loaded when SharedConfigEnable set.
aws/session: Fix env vars being correctly loaded when SharedConfigEnable set.

The AWS_DEFAULT_PROFILE was not loaded if SharedConfigState = SharedConfigEnable.  This changes fixes the loading of the env vars for shared config to be used when enabled in the session options.

Fix #849
xibz/aws-sdk-go,calgaryscientific/aws-sdk-go,agworld/aws-sdk-go,xibz/aws-sdk-go,calgaryscientific/aws-sdk-go,aws/aws-sdk-go,radeksimko/aws-sdk-go,agworld/aws-sdk-go,xibz/aws-sdk-go,zmarois/aws-sdk-go,radeksimko/aws-sdk-go,myshen/aws-sdk-go,yxd-hde/aws-sdk-go,calgaryscientific/aws-sdk-go,myshen/aws-sdk-go,calgaryscientific/aws-sdk-go,agworld/aws-sdk-go,radeksimko/aws-sdk-go,myshen/aws-sdk-go,radeksimko/aws-sdk-go,jasdel/aws-sdk-go,yxd-hde/aws-sdk-go,yxd-hde/aws-sdk-go,zmarois/aws-sdk-go,yxd-hde/aws-sdk-go,zmarois/aws-sdk-go,myshen/aws-sdk-go,zmarois/aws-sdk-go,agworld/aws-sdk-go,xibz/aws-sdk-go