Fix JSON serialization of message attributes with a custom data type

[mgocd]

Closes #947

Additional info:

• Added custom data type to existing tests in AmazonJavaSdkTestSuite - note the tests will succeed also without changes in MessageAttributesSupport , because aws-java-sdk-sqs version used is 1.12.472 (pre-1.12.584 when JSON protocol was introduced)
• Added additional tests to AmazonJavaSdkV2TestSuite basing on AmazonJavaSdkTestSuite (some of these tests will not succeed without changes in MessageAttributesSupport)
• The new tests also are able to reproduce the issue described in https://github.com/softwaremill/elasticmq/issues/946, occurring when a binary message attribute is used - need to uncomment "green" -> BinaryMessageAttribute("dog".getBytes("UTF-8")) lines in AmazonJavaSdkV2TestSuite. This PR does not fix this issue.

[mgocd]

Moved to a different fork: https://github.com/softwaremill/elasticmq/pull/950