fix: set accept header only if not set request headers

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+- Do not set `accept` header when defined in [HTTP request headers](https://spec.superface.dev/latest/map-spec.html#HTTPHeaders)
+
 ## [2.0.0] - 2022-08-15
 ### Added
 - Pass `cache` flag to SuperfaceClient constructor

src/core/interpreter/http/filters.test.ts

@@ -360,5 +360,22 @@ describe('HTTP Filters', () => {
         })
       );
     });
+
+    it("doesn't set accept header if already exists", async () => {
+      const result = await headersFilter({
+        parameters: {
+          url: '/test',
+          method: '',
+          baseUrl: 'https://example.com',
+          headers: {
+            aCcEpT: 'application/json',
+          },
+          accept: 'text/plain',
+          contentType: JSON_CONTENT,
+        },
+      });
+
+      expect(result.request?.headers?.accept).toBe(undefined);
+    })
   });
 });

src/core/interpreter/http/filters.ts

@@ -26,7 +26,7 @@ import type {
   RequestParameters,
 } from './security';
 import type { HttpResponse } from './types';
-import { createUrl, fetchRequest } from './utils';
+import { createUrl, fetchRequest, hasAcceptHeader } from './utils';
 
 /**
  * Represents input of pipe filter which works with http response
@@ -258,8 +258,12 @@ export const headersFilter: Filter = ({
   request,
   response,
 }: FilterInputOutput) => {
-  const headers: Record<string, string> = parameters.headers || {};
-  headers['accept'] = parameters.accept ?? '*/*';
+  const headers: Record<string, string> = parameters.headers ?? {};
+
+  if (!hasAcceptHeader(headers)) {
+    headers['accept'] = parameters.accept ?? '*/*';
+  }
+
   headers['user-agent'] ??= USER_AGENT;
   if (parameters.contentType === JSON_CONTENT) {
     headers['content-type'] ??= JSON_CONTENT;

src/core/interpreter/http/utils.ts

@@ -140,3 +140,9 @@ export async function fetchRequest(
     },
   };
 }
+
+export function hasAcceptHeader(headers: NonPrimitive): boolean {
+  return Object.keys(headers).some(
+    header => header.toLowerCase() === 'accept'
+  );
+}

src/core/interpreter/map-interpreter.ts

@@ -327,7 +327,7 @@ export class MapInterpreter<TInput extends NonPrimitive | undefined>
       this.visit(responseHandler)
     );
 
-    let accept = '';
+    let accept: string;
     if (responseHandlers.some(([, accept]) => accept === undefined)) {
       accept = '*/*';
     } else {