Fix passing of kwargs to init_app

[jethroguce]

initiating app instance with arguments does not take effect,

api = Api(app, add_specs=False)

but when using the lazy instantiation it works

api = Api()
api.init_app(app, add_specs=False)

[coveralls]

Coverage decreased (-0.07%) to 96.837% when pulling 888b9ae on jethroguce:master into a7e363a on noirbizarre:master.

[jethroguce]

@j5awry can you have a quick look at this

[j5awry]

1. we should check the tests for coverage of this new call
2. looking at the history, git blame, and the commented portion, it looks like this capability existed before. Looks like 4 years ago there was a switch, and the old way did pass kwargs to a parent class (that looks like it did do what this is asking for)

From that standpoint, it's worth asking if this is meant to, since it seems to be a conscious change. I'm in favor of passing kwargs down though.

[SteadBytes]

I'm also in favour of passing kwargs down, however it would certainly be useful to understand the original reason for removing it if possible.

2 things before merging if you don't mind:

1. Cover the change with tests
2. Remove https://github.com/noirbizarre/flask-restplus/pull/722/files#diff-31928362e95b5b97b24d2ccf01506003R153 - may as well clean it up a bit whilst here 😄