awsRegion does not override awsOptions.region

[Nalgmann]

Issue type: Bug

Hi.
According with docs, awsOptions are overridden by any other if specified. This is not true in case of awsRegion.
This is why:

index.js, lines 42-55:

 var config = {};

  if (awsAccessKeyId && awsSecretKey && awsRegion) {
    config = { accessKeyId: awsAccessKeyId, secretAccessKey: awsSecretKey, region: awsRegion };
  } else if (awsRegion && !awsAccessKeyId && !awsSecretKey) {
    // Amazon SDK will automatically pull access credentials
    // from IAM Role when running on EC2 but region still
    // needs to be configured
    config = { region: awsRegion };
  }

if (options.awsOptions) {
    config = assign(config, options.awsOptions);
  }

In line 54 occurs everything. According to lodash: Subsequent sources overwrite property assignments of previous sources.

This may cause confusion using the package.

A possible solution may be assigning options.awsOptions before awsAccessKeyId, awsSecretKey and awsRegion. When assigning this three properties use assign()