Fix merging custom headers (#36)

Author: dillonredding

src/submit.spec.ts

@@ -101,16 +101,34 @@ describe('submit', () => {
     expect(scope.isDone()).toBe(true);
   });
 
-  it('should accept and send request options', async () => {
+  describe('requestInit option', () => {
     const apiKey = 'foo-bar-baz';
     const headers = { 'Api-Key': apiKey };
-    const scope = nock(baseUrl, { reqheaders: headers }).get(path).reply(204);
+    const action = new Action();
+    action.name = 'foo';
+    action.href = url;
+    action.method = 'POST';
+    action.fields = [nameField];
 
-    const response = await submit(action, { requestInit: { headers } });
+    it('should send custom headers', async () => {
+      const scope = nock(baseUrl, { reqheaders: headers }).post(path).reply(204);
 
-    expect(response.url).toBe(url);
-    expect(response.status).toBe(204);
-    expect(scope.isDone()).toBe(true);
+      const response = await submit(action, { requestInit: { headers } });
+
+      expect(response.url).toBe(url);
+      expect(response.status).toBe(204);
+      expect(scope.isDone()).toBe(true);
+    });
+
+    it('should merge custom headers with base headers', async () => {
+      const scope = nock(baseUrl, { reqheaders: headers }).post(path).reply(204);
+
+      const response = await submit(action, { requestInit: { headers: Object.entries(headers) } });
+
+      expect(response.url).toBe(url);
+      expect(response.status).toBe(204);
+      expect(scope.isDone()).toBe(true);
+    });
   });
 
   it('should resolve relative URL', async () => {

src/submit.ts

@@ -1,4 +1,4 @@
-import fetch from 'cross-fetch';
+import { fetch, Headers } from 'cross-fetch';
 
 import { Href } from './href';
 import { Action } from './models/action';
@@ -52,7 +52,9 @@ export async function submit(action: Action, options: SubmitOptions = {}): Promi
       target.search = serialization.content.toString();
     } else {
       if (serialization.contentType) {
-        init.headers = { ...init.headers, 'Content-Type': serialization.contentType };
+        const headers = new Headers(init.headers);
+        headers.set('Content-Type', serialization.contentType);
+        init.headers = headers;
       }
       init.body = serialization.content;
     }