In the case where a TomlField is being applied to a Dictionary with enum keys, there is a possibility for an exception to be thrown that can't be handled well if an invalid enum key exists in the TOML data.

Reproduction:

(withIgnoreInvalidEnumValues set to true)

a. Parse a TOML string with multiple invalid keys
b. Parse a TOML string with one invalid key, and at least one other valid key at the default value

Result:

a. An key is added at the default (0) value with the invalid enum's value, which is hard to detect
b. An exception is thrown when a valid key at the default (0) value also exists
c. An exception is thrown when multiple invalid keys are found

Expected:

Invalid keys should not be added to the dictionary, multiple keys should never happen

Changes in this PR:

• Invalid keys are no longer added to the resulting table.
• Unit tests have also been added to ensure the new behaviour works, and that it doesn't break the behaviour of multiple keys being illegal under the TOML standard.