Fix errors in Kotlin Unit methods (#1124)

Kotlin API fails on methods that don't return anything (which currently
just includes `delete` methods) because the generated code by default
tries to serialize the methods' return types in all cases, even if the
return type is `Unit`. I'm not exactly sure whether this is properly a
bug in the openapi generator or in moshi. In any case, I was unable to
fix the error by doing what initially seemed most sensible, which was to
register a moshi adapter for the `Unit` type.

The fix here is not terribly elegant but in practice seems to fix the
issue. I will also see about opening/adding to an existing issue on the
openapi generator's github page and/or submitting a PR to fix this
upstream.

Note also that I've removed the `Serializer.kt` template, which is
unused. We've already got an actual `Serializer` class stored in the
generated code folder, so there's no reason to have the template.

Fixes #1117.

kotlin/lib/src/test/com/svix/kotlin/BasicTest.kt

@@ -24,6 +24,7 @@ class BasicTest {
                     )
                 )
             )
+            svix.application.delete(applicationOut.id)
         }
     }
 

kotlin/templates/libraries/jvm-okhttp/infrastructure/ApiClient.kt.mustache

@@ -171,9 +171,16 @@ import kotlin.random.Random
             )
             {{/jvm-okhttp3}}
             {{#jvm-okhttp4}}
-            mediaType == JsonMediaType -> {{#moshi}}Serializer.moshi.adapter(T::class.java).toJson(content){{/moshi}}{{#gson}}Serializer.gson.toJson(content, T::class.java){{/gson}}{{#jackson}}Serializer.jacksonObjectMapper.writeValueAsString(content){{/jackson}}{{#kotlinx_serialization}}Serializer.jvmJson.encodeToString(content){{/kotlinx_serialization}}.toRequestBody(
-                mediaType.toMediaTypeOrNull()
-            )
+            mediaType == JsonMediaType -> {
+                // Don't call `toJson` on Unit type:
+                if (T::class.java == Unit::class.java) {
+                    ByteArray(0).toRequestBody()
+                } else {
+                    {{#moshi}}Serializer.moshi.adapter(T::class.java).toJson(content){{/moshi}}{{#gson}}Serializer.gson.toJson(content, T::class.java){{/gson}}{{#jackson}}Serializer.jacksonObjectMapper.writeValueAsString(content){{/jackson}}{{#kotlinx_serialization}}Serializer.jvmJson.encodeToString(content){{/kotlinx_serialization}}.toRequestBody(
+                        mediaType.toMediaTypeOrNull()
+                    )
+                }
+            }
             {{/jvm-okhttp4}}
             mediaType == XmlMediaType -> throw UnsupportedOperationException("xml not currently supported.")
             // TODO: this should be extended with other serializers