Enum documentation is incorrect when value is unknown and dest is set

[Woodz]

The documentation for the default value reads Default value to be used for all values not contained in the mapping table. When unset, the unmodified value for the field will be used if no match is found.

However, if default value is unset and dest is set, the unmodified value is not used - the dest field is missing.

Version: 0a4d74c

Reproducible:

Configuration:

[[processors.enum]]
[[processors.enum.mapping]]
## Name of the field to map
field = "status"
## Name of the tag to map
# tag = "status"
## Destination tag or field to be used for the mapped value. By default the 
## source tag or field is used, overwriting the original value.
dest = "status_code"
## Default value to be used for all values not contained in the mapping
## table. When unset, the unmodified value for the field will be used if no
## match is found.
# default = 0
## Table of mappings
[processors.enum.mapping.value_mappings]
green = 1
amber = 2
red = 3 

Expected (based on doc):

- xyzzy status="black" 1502489900000000000
+ xyzzy status="black",status_code="black" 1502489900000000000

Actual:

- xyzzy status="black" 1502489900000000000
+ xyzzy status="black" 1502489900000000000

Now, I don't think the expected is sensible, but there is no unit test to show the correct expected behaviour, but my recommendation would be to correct the documentation and add a unit test to match the current implementation.