Signature helper fails with Encoding::UndefinedConversionError

[Sokre95]

When using Hubspot::Helpers::Signature to verify webhooks Encoding::UndefinedConversionError is raised when payload contains characters like ü.

Error is raised when encode is called on source_string

Error message:

Encoding::UndefinedConversionError: "\xC3" from ASCII-8BIT to UTF-8

Actual error can occur for all 3 signature versions. I hot-fixed this in our fork by calling force_encoding, but I'm afraid that's not the proper fix.