only rename existing fields and keep the order

[Rafaellinos]

Changes effects:

• Fixes issue Using rename fields changes the key order of the log output #169
• Keep the original order when renaming fields;
• Ignore nonexisting fields, avoiding fields with empty values;
• No more raises an error while trying to rename non existing fields.

[vpmedia]

@madzak any chance reviewing this in the near future?

[akullenberg-cbp]

@madzak It would be great to see this merged...

[nhairs]

Hi @Rafaellinos,

I'd like to add this change in the fork I've made of pyton-json-logger. Would you be willing to open a PR there?