fix: do not override content-type if its given (#158)

* fix: do not override content-type if its given

* gts fix

src/gaxios.ts

@@ -187,7 +187,13 @@ export class Gaxios {
         opts.body = opts.data;
       } else if (typeof opts.data === 'object') {
         opts.body = JSON.stringify(opts.data);
-        opts.headers['Content-Type'] = 'application/json';
+        if (
+          !Object.keys(opts.headers).some((key: string) =>
+            key.match(/^content-type$/i)
+          )
+        ) {
+          opts.headers['Content-Type'] = 'application/json';
+        }
       } else {
         opts.body = opts.data;
       }

test/test.getch.ts

@@ -292,6 +292,37 @@ describe('🎏 data handling', () => {
     assert.deepStrictEqual(res.data, {});
   });
 
+  it('should set content-type for object request', async () => {
+    const body = {hello: '🌎'};
+    const scope = nock(url)
+      .matchHeader('content-type', 'application/json')
+      .post('/', JSON.stringify(body))
+      .reply(200, {});
+    const res = await request({
+      url,
+      method: 'POST',
+      data: body,
+    });
+    scope.done();
+    assert.deepStrictEqual(res.data, {});
+  });
+
+  it('should allow to override content-type for object request', async () => {
+    const body = {hello: '🌎'};
+    const scope = nock(url)
+      .matchHeader('content-type', 'application/octet-stream')
+      .post('/', JSON.stringify(body))
+      .reply(200, {});
+    const res = await request({
+      url,
+      method: 'POST',
+      data: body,
+      headers: {'content-type': 'application/octet-stream'},
+    });
+    scope.done();
+    assert.deepStrictEqual(res.data, {});
+  });
+
   it('should return stream if asked nicely', async () => {
     const body = {hello: '🌎'};
     const scope = nock(url)