fix: parse JSON body when Content-Type has charset

Or other parameters.

In accordance with RFC 1341[^1], Content-Type contains first and
foremost the media type. It can be followed by key-value pairs of
parameters.

This change supports extracting the media type and effectively
discarding the other parameters as these are currently irrelevant to
body parsing. The result media type is then matched against previously
used JSON type tests.

Closes apollographql#229
Closes apollographql#341

[^1]:https://www.rfc-editor.org/rfc/rfc1341

.changeset/unlucky-geese-give.md

@@ -0,0 +1,5 @@
+---
+'@apollo/datasource-rest': patch
+---
+
+Parse JSON body when Content-Type has charset

src/RESTDataSource.ts

@@ -303,14 +303,14 @@ export abstract class RESTDataSource<CO extends CacheOptions = CacheOptions> {
   protected parseBody(response: FetcherResponse): Promise<object | string> {
     const contentType = response.headers.get('Content-Type');
     const contentLength = response.headers.get('Content-Length');
+    const mediaType = contentType?.split(';')?.[0];
     if (
       // As one might expect, a "204 No Content" is empty! This means there
       // isn't enough to `JSON.parse`, and trying will result in an error.
       response.status !== 204 &&
       contentLength !== '0' &&
-      contentType &&
-      (contentType.startsWith('application/json') ||
-        contentType.endsWith('+json'))
+      (mediaType?.startsWith('application/json') ||
+        mediaType?.endsWith('+json'))
     ) {
       return response.json();
     } else {

src/__tests__/RESTDataSource.test.ts

@@ -784,6 +784,50 @@ describe('RESTDataSource', () => {
         expect(data).toEqual({ foo: 'bar' });
       });
 
+      it('returns data as parsed JSON when Content-Type ends in +json and has charset provided', async () => {
+        const dataSource = new (class extends RESTDataSource {
+          override baseURL = 'https://api.example.com';
+
+          getFoo() {
+            return this.get('foo');
+          }
+        })();
+
+        nock(apiUrl)
+          .get('/foo')
+          .reply(
+            200,
+            { foo: 'bar' },
+            { 'content-type': 'application/vnd.api+json; charset=utf-8' },
+          );
+
+        const data = await dataSource.getFoo();
+
+        expect(data).toEqual({ foo: 'bar' });
+      });
+
+      it('returns data as parsed JSON when Content-Type ends in +json and has many parameters provided', async () => {
+        const dataSource = new (class extends RESTDataSource {
+          override baseURL = 'https://api.example.com';
+
+          getFoo() {
+            return this.get('foo');
+          }
+        })();
+
+        nock(apiUrl)
+          .get('/foo')
+          .reply(
+            200,
+            { foo: 'bar' },
+            { 'content-type': 'application/vnd.api+json; charset=utf-8; boundary=ExampleBoundaryString' },
+          );
+
+        const data = await dataSource.getFoo();
+
+        expect(data).toEqual({ foo: 'bar' });
+      });
+
       it('returns data as a string when Content-Type is text/plain', async () => {
         const dataSource = new (class extends RESTDataSource {
           override baseURL = 'https://api.example.com';