Maintain order in which methods are added to endpoint

[lafrech]

Similar to #86 and #87.

[lafrech]

Rebasing this on #190 assuming it would be merged as is. If not, I'll rebase again.

This change is useful to me because I'm enforcing the order of the methods for each endpoint in the spec:

['OPTIONS', 'HEAD', 'GET', 'POST', 'PUT', 'PATCH', 'DELETE', 'TRACE']

I find it nice to have a consistent method order.

It works because I'm iterating on the methods in my lib, not in apispec.

---

For people using apispec.ext.flask.path_from_view, it would take another modification in there:

HTTP_METHODS = ['OPTIONS', 'HEAD', 'GET', 'POST', 'PUT', 'PATCH', 'DELETE', 'TRACE']

def path_from_view(spec, view, **kwargs):
    [...]
    operations = OrderedDict()
    for method in HTTP_METHODS:
        if method in view.methods and method in rule.methods:
            [...]

Likewise for tornado and the others.

This means hardcoding the method order. The user could mutate the global method list to control the order, though.

I think this can be done in another PR without holding this one.

[sloria]

Thanks @lafrech . #190 is merged, but it looks like this needs to be rebased one more time. Also, would you mind opening an issue for the Flask/Tornado issue you mentioned?

[lafrech]

Great. I just rebased and opened #193.