Updating with saveUnknown enabled

[fishcharlie]

Summary:

This PR is adding a test for a related issue that needs to be fixed. Currently updating with saveUnknown enabled fails silently, and doesn't update with unknown properties.

GitHub linked issue:

#403

Type (select 1):

• Bug fix
• Feature implementation
• Documentation improvement
• Testing improvement
• Test added to report bug (GitHub issue Updating with saveUnknown Fails #403 @fishcharlie)
• Something not listed here

Is this a breaking change? (select 1):

• 🚨 YES 🚨
• No
• I'm not sure

Is this ready to be merged into Dynamoose? (select 1):

• Yes
• No

Other:

• I have searched through the GitHub pull requests to ensure this PR has not already been submitted
• I have updated the Dynamoose documentation (if required) given the changes I made
• I have added/updated the Dynamoose test cases (if required) given the changes I made
• I have run npm test from the root of the project directory to ensure all tests continue to pass
• I agree that all changes made in this pull request may be distributed and are made available in accordance with the Dynamoose license
• All of my commits and commit messages are detailed, explain what changes were made, and are easy to follow and understand
• I have filled out all fields above