Have AWSCredentialsUtils.load() perform the check for STS credentials

[JoshRosen]

This patch moves the check for STS credentials into the AWSCredentials.load() call itself. The motivation for this is a single call-site where we forgot to do a .getOrElse(), which caused the STS credentials to be ignored. This broke spark-redshift for users who authenticated to AWS only via IAM instance profiles.

[JoshRosen]

This is the callsite which was missing the params.temporaryAWSCredentials.getOrElse() call.

[codecov-io]

Current coverage is 88.97%

Merging #159 into master will decrease coverage by -0.02% as of b9fa8e2

@@            master    #159   diff @@
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  Files           13      13       
  Stmts          645     644     -1
  Branches       142     142       
  Methods          0       0       
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- Hit            574     573     -1
  Partial          0       0       
  Missed          71      71       

Review entire Coverage Diff as of b9fa8e2

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