Add support for plain/text response type

[JKostov]

Fixes #376

[Alan-Cha]

I need to take a closer look later but I believe this is supposed to handle the case of non-application/json.

[JKostov]

@Alan-Cha
The problem is when there is no schema in the response type (like the example below) then the responseSchema is undefined on this line and then we get the error from issue #376.

paths:
  "/test":
    get:
      summary: Get string value
      responses:
        "200":
          description: Returns value as a string
          content:
            text/plain:
              example: example

[Alan-Cha]

I see! In that case, could we just reorganize the code so that we still use this but it checks if a responseSchema exists first?

[JKostov]

Yes, it should fix the issue.

[Alan-Cha]

@JKostov I've addressed this with #391. Thank you for all your help!