api: add upsert entrypoint for route updates

[ucirello]

Currently we can only issue POST (create), DELETE and and GET to Routes. So whenever faced with the need to update a route, we are forced to do a delete-then-create for the routes; triggering service disruption during interfaces.

fnctl takes this approach for now - and fixing this issue must also change its source code.

It relates to #259 and its related issues.

[ucirello]

cc @pedronasser

[ucirello]

cc @denismakogon

[ucirello]

Commenting these lines out: https://github.com/iron-io/functions/blob/master/fnctl/routes.go#L340-L342

And running:

./fnctl routes config set myapp hello key1 value1
./fnctl routes config set myapp hello key2 value2

the key2/value2 pair will not be there.

In the logs, you should get a "error=Route already exists"