Allow reopening of docs definition for the swagger_api DSL command

This allows for some interesting inheritance mechanics, like defining the API
for a controller method in a superclass and then refining the API method, and
therefore documentation, in the subclass controller.

Signed-off-by: Lucas Dutra Nunes <ldnunes@ossystems.com.br>
Merge pull request #44 from ldnunes/allow_docs_reopening

Allow reopening of docs definition for the swagger_api DSL command