* Allow numeric and non-string values for tag_keys.

According to the go documentation the JSON deserializer only produces these
base types in output:
- string
- bool
- float64
- nil
With this patch bool, float64 and nil values get converted to a string when
their field key is specified in tag_keys. Previously the field was simply
discarded.

* Updated handling of nil for passing tests.

The automated tests are less than trivial to reproduece locally for me,
so I hope CircleCI wonn't mind...

* Updated changelog entries with PR and issue links.