Fix for Aws::S3::Client#head_bucket with improper region.

Resolved the following issues:

  s3 = Aws::S3::Client.new(region: 'us-west-1')
  s3.head_bucket(bucket: 'us-west-2-bucket')
  #=> raises Aws::S3::Errors::BadRequest (without a message)

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 results in an empty error message. Added support for loading the
bucket region from the "x-amz-bucket-region" header, which is necessary
for all HEAD responses.

Fixes #1161

aws-sdk-core/features/s3/buckets.feature

@@ -9,6 +9,12 @@ Feature: S3 Buckets
     When I delete the bucket
     Then the bucket should not exist
 
+  Scenario: HEAD bucket works with improper region
+    Given I am using the S3 "us-west-2" region
+    And I create a bucket
+    When I am using the S3 "us-east-1" region
+    Then I should be able to HEAD the bucket
+
   Scenario: CRUD buckets using a regional endpoint
     Given I am using the S3 "us-west-2" region
     When I create a bucket

aws-sdk-core/features/s3/step_definitions.rb

@@ -61,6 +61,10 @@ def create_bucket(options = {})
   expect { @client.get_bucket_location(bucket: @bucket_name) }.not_to raise_error
 end
 
+Then(/^I should be able to HEAD the bucket$/) do
+  expect { @client.head_bucket(bucket: @bucket_name) }.not_to raise_error
+end
+
 When(/^I delete the bucket$/) do
   @client.delete_bucket(bucket: @bucket_name)
   @created_buckets.delete(@bucket_name)

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)