fix: Fixing issues in adding number parameters (#364)

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# Closes #
[DII-792](https://issues.corp.twilio.com/browse/DII-792) Integer
conversion is not proper in v6 of twilio-csharp library
A short description of what this PR does.
Integer conversion is not proper in v6 of twilio-csharp library
### Checklist
- [x] I acknowledge that all my contributions will be made under the
project's license
- [ ] Run `make test-docker`
- [ ] Verify affected language:
- [ ] Generate [twilio-go](https://github.com/twilio/twilio-go) from our
[OpenAPI specification](https://github.com/twilio/twilio-oai) using the
[build_twilio_go.py](./examples/build_twilio_go.py) using `python
examples/build_twilio_go.py path/to/twilio-oai/spec/yaml
path/to/twilio-go` and inspect the diff
    - [ ] Run `make test` in `twilio-go`
    - [ ] Create a pull request in `twilio-go`
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and my PR follows them
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- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have added the necessary documentation about the functionality
in the appropriate .md file
- [ ] I have added inline documentation to the code I modified

If you have questions, please create a GitHub Issue in this repository.

Author: AsabuHere

examples/csharp/src/Twilio/Rest/FlexApi/V1/Credential/NewCredentialsOptions.cs

@@ -110,7 +110,7 @@ public  List<KeyValuePair<string, string>> GetParams()
             }
             if (TestNumber != null)
             {
-                p.Add(new KeyValuePair<string, string>("TestNumber", TestNumber.ToString()));
+                p.Add(new KeyValuePair<string, string>("TestNumber", TestNumber.Value.ToString()));
             }
             if (TestNumberFloat != null)
             {

src/main/resources/config/csharp.json

@@ -117,6 +117,7 @@
         "object": "Serializers.JsonObject(%s)",
         "bool?": "%s.Value.ToString().ToLower()",
         "decimal": "%s.Value.ToString()",
+        "decimal?": "%s.Value.ToString()",
         "int?": "%s.ToString()",
         "long?": "%s.Value.ToString()"
     },