fix: Support Decimal for tag_object

[astuyve]

What does this PR do?

decimal.Decimal is an object, but that throws an error as it's not iterable.
This PR fixes that.

Motivation`

Fixes #213

Testing Guidelines

Covered by automated tests.

Additional Notes

Types of Changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)

[hghotra]

👍🏼