Tighten up thenReply valid headers for trailers check

Any set value is actually sufficient to cause issues - even just {}
disables the default headers, which makes trailers unsendable.

Author: pimterry

src/rules/requests/request-handler-definitions.ts

@@ -279,7 +279,7 @@ export class SimpleHandlerDefinition extends Serializable implements RequestHand
         validateCustomHeaders({}, headers);
         validateCustomHeaders({}, trailers);
 
-        if (!_.isEmpty(trailers) && !_.isEmpty(headers)) {
+        if (!_.isEmpty(trailers) && headers) {
             if (!Object.entries(headers!).some(([key, value]) =>
                 key.toLowerCase() === 'transfer-encoding' && value === 'chunked'
             )) {

test/integration/subscriptions/response-events.spec.ts

@@ -171,7 +171,7 @@ describe("Response subscriptions", () => {
         });
 
         it("should include raw trailer data", async () => {
-            await server.forGet('/mocked-endpoint').thenReply(200, undefined, {}, {
+            await server.forGet('/mocked-endpoint').thenReply(200, undefined, undefined, {
                 "custom-TRAILER": "TRAILER-value"
             });