Fix for Aws::S3::Bucket#exists?

Fixed the following issue:

  s3 = Aws::S3::Resource.new(region: 'us-west-1')

  s3.bucket('us-west-2-bucket-you-dont-own').exists?
  #=> raises Aws::S3::Errors::BadRequest

If the bucket exists, but you are configured for the incorrect region,
the SDK fails to extract the actual bucket region from the 400 response.
This is due to the fact that HEAD reponses do not have a body.

Added a check for the "x-amz-bucket-region" header before checking the
body. This the SDK to determine the proper signing region for the
request.

This change currently lacks tests.

See #1161

aws-sdk-core/lib/aws-sdk-core/plugins/s3_request_signer.rb

@@ -100,10 +100,8 @@ def handle_region_errors(response)
         end
 
         def get_region_and_retry(context)
-          actual_region = region_from_body(context)
-          if actual_region.nil? || actual_region == ""
-            raise "Couldn't get region from body: #{context.body}"
-          end
+          actual_region = context.http_response.headers['x-amz-bucket-region']
+          actual_region ||= region_from_body(context.http_response.body_contents)
           update_bucket_cache(context, actual_region)
           log_warning(context, actual_region)
           update_region_header(context, actual_region)
@@ -116,7 +114,10 @@ def update_bucket_cache(context, actual_region)
 
         def wrong_sigv4_region?(resp)
           resp.context.http_response.status_code == 400 &&
+          (
+            resp.context.http_response.headers['x-amz-bucket-region'] ||
             resp.context.http_response.body_contents.match(/<Region>.+?<\/Region>/)
+          )
         end
 
         def update_region_header(context, region)
@@ -128,8 +129,13 @@ def update_region_header(context, region)
           signer.sign(context.http_request)
         end
 
-        def region_from_body(context)
-          context.http_response.body_contents.match(/<Region>(.+?)<\/Region>/)[1]
+        def region_from_body(body)
+          region = body.match(/<Region>(.+?)<\/Region>/)[1]
+          if region.nil? || region == ""
+            raise "couldn't get region from body: #{body}"
+          else
+            region
+          end
         end
 
         def log_warning(context, actual_region)