Creates a new credentials object.
@param (see AWS.STS.assumeRoleWithSAML)
@example Creating a new credentials object
AWS.config.credentials = new AWS.SAMLCredentials({
RoleArn: 'arn:aws:iam::1234567890:role/SAMLRole',
PrincipalArn: 'arn:aws:iam::1234567890:role/SAMLPrincipal',
SAMLAssertion: 'base64-token', // base64-encoded token from IdP
});
@see AWS.STS.assumeRoleWithSAML
function SAMLCredentials(params) {
    AWS.Credentials.call(this);
    this.expired = true;
    this.params = params;
    this.service = new AWS.STS({params: this.params});
  }