Extensions in subobjects of OpenAPI no longer work

[ezoerner]

This is closely related to #2104, which states that extensions in the OpenAPI object no longer work.
This was fixed for extensions at the top level, but extension in sub-parts of OpenAPI are also no longer working. One of these sub-objects is the Info object which can also have extensions. I believe there are extensions in other sub-parts that are probably also not working.

I've verified that the extensions of Info are not working in v1.6.8, 1.16.15, and 1.7.0.

[bnasslahsen]

@ezoerner,

I have added a fix for it.
Let me know if the fix works for you with the latest snapshot.