protoc-gen-swagger: Fix message recursion detection

[doyleish]

• Fixes a bug introduced during 9e25b2b which added recursion
  detection within messages. Briefly, the nature of the bug was that
  references to messages not in the current recursion traversal were
  remaining in the map used to track those occurrences.
• Includes a test case outlining the edge case that prompted this fix.

References to other Issues or PRs

Have you read the Contributing Guidelines?

Brief description of what is fixed or changed

Briefly, the nature of the bug was that references to messages not in the current recursion traversal path were remaining in the map used to track those occurrences. The fix just ensures those references don't hang around while recursing and are passed by value only.

Other comments

Testing the new case before fix...

(protoc-gen-swagger:)[16:32] ryan@psyduck ✘+ */grpc-gateway/protoc-gen-swagger $> go test ./genswagger/                                                                     
--- FAIL: TestMessageToQueryParametersNoRecursive (0.00s)
    template_test.go:518: No recursion error should be thrown: Recursive types are not allowed for query parameters, cycle found on ".example.NonRecursiveMessage"

And after the fix...

[1](protoc-gen-swagger:)[16:32] ryan@psyduck ✘+ */grpc-gateway/protoc-gen-swagger $> go test ./genswagger/
ok      github.com/grpc-ecosystem/grpc-gateway/protoc-gen-swagger/genswagger    0.005s

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
In order to pass this check, please resolve this problem and then comment @googlebot I fixed it.. If the bot doesn't comment, it means it doesn't think anything has changed.

ℹ️ Googlers: Go here for more info.

[doyleish]

@googlebot I fixed it.

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[johanbrandhorst]

Brilliant! This looks great, sorry for the inconvenience.

[johanbrandhorst]

Fuzzit is being flukey, I'll override it

[johanbrandhorst]

Thanks for your contribution! Could you please cherry pick this against v2?

[doyleish]

Thanks @johanbrandhorst for the merge!
I've created #1439 cherry-picking this to v2