Separate path_helper and operation_helper logic in web framework plugins

[lafrech]

Issue by sloria Monday Jul 30, 2018 at 00:44 GMT Originally opened as https://github.com/marshmallow-code/apispec/issues/257

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From #246:

Also the web framework path helpers deal with operations quite a lot, and perhaps this logic should be split into path_helper and operation_helper. From a quick glance, the coupling between the two seems loose. Could be a bit trickier with flask due to the if method in rule.methods test, which covers what looks like a twisted but legit case (method declared in the view but not in the rule).