LTSV decoder doesn't respect output.ltsv_extra add or replace fields

[kurojishi]

Found this while writing tests

As for https://github.com/jedisct1/flowgger/wiki/LTSV-Encoder the encoders should support the ability to add or replace, without duplicating tags and fields.

LTSV is not only not overriding it, but get duplicated as the encoding is done by appending data to a String:

https://github.com/awslabs/flowgger/blob/master/src/flowgger/encoder/ltsv_encoder.rs#L41

left: "some_info:foo\tsome_info:bar\thost:example.org\ttime:1385053862.3072\tmessage:A short message\tfull_message:Backtrace here with more\tlevel:1\tappname:appname\tprocid:44\tmsgid:msg_id",
right: "some_info:bar\thost:example.org\ttime:1385053862.3072\tmessage:A short message\tfull_message:Backtrace here with more\tlevel:1\tappname:appname\tprocid:44\tmsgid:msg_id"', src/flowgger/encoder/ltsv_encoder.rs:175:9