Parsing HEC's UseACK by int, bool and string

[spurge]

Resolves #130 by parsing multiple different types of data in the useACKfield.

In our Cloud environment, this field comes as a stringified boolean, such as a "false". Which caused an error as the string wasn't able to be parsed into an int.

These changes will cope with useACK as both int and bool by using a custom json-marshaller ...

• int as a string: "1"
• int as an int: 1
• bool as a string: "true"
• bool as a bool: true

There's test cases for all these variants.

[spurge]

Splunk provides another Terraform Provider for it's cloud platform.