Don't set the content-type when there is no payload

The HTTP Get operation actually set a content-type of 'application/json' when there is no payload. The request adds a content-length of 0 because there is no payload.

Web server like https://github.com/stoplightio/prism (mock of openapi) reject the calls because the content-type has nothing to do with GET operations.

Exemple:
GET http://127.0.0.1:4010/RESTMethod HTTP/1.1
content-type: application/json
accept: application/json
host: 127.0.0.1:4010
content-length: 0
Connection: close

Author: Lionel Schiepers

packages/openapi-to-graphql/src/resolver_builder.ts

@@ -446,10 +446,14 @@ export function getResolver({
      *
      * NOTE: This may cause the user to encounter unexpected changes
      */
-    headers['content-type'] =
-      typeof operation.payloadContentType !== 'undefined'
-        ? operation.payloadContentType
-        : 'application/json'
+    if (operation.payloadRequired)
+    {
+      headers['content-type'] =
+        typeof operation.payloadContentType !== 'undefined'
+          ? operation.payloadContentType
+          : 'application/json'
+    }
+
     headers['accept'] =
       typeof operation.responseContentType !== 'undefined'
         ? operation.responseContentType