Fix FormData to properly handle appended arrays. (facebook#32815)

Summary:
The Array appended to FormData must be transmitted in the form of a string.
However, it is treated as a file object and transmitted, because `typeof Array` is `'object'` too

In network
```js
form.append('array_name', ['a', 'b', 'c'])

// Browser
// Content-Disposition: form-data; name='array_name';
// a,b,c

// ReactNative
// Content-Disposition: form-data; name='array_name';
//
```

## Changelog
[General] [Fixed] - The Array appended to FormData is transmitted as a string

Pull Request resolved: facebook#32815

Test Plan: Added test case

Reviewed By: lunaleaps

Differential Revision: D33369594

Pulled By: charlesbdudley

fbshipit-source-id: 0b5219a2c9f73cf16665dc417cceb4481428ad4e

Libraries/Network/FormData.js

@@ -74,7 +74,7 @@ class FormData {
       // an object with a `uri` attribute. Optionally, it can also
       // have a `name` and `type` attribute to specify filename and
       // content type (cf. web Blob interface.)
-      if (typeof value === 'object' && value) {
+      if (typeof value === 'object' && !Array.isArray(value) && value) {
         if (typeof value.name === 'string') {
           headers['content-disposition'] += '; filename="' + value.name + '"';
         }

Libraries/Network/__tests__/FormData-test.js

@@ -55,4 +55,26 @@ describe('FormData', function () {
     };
     expect(formData.getParts()[0]).toMatchObject(expectedPart);
   });
+
+  it('should return non blob array', function () {
+    formData.append('array', [
+      true,
+      false,
+      undefined,
+      null,
+      {},
+      [],
+      'string',
+      0,
+    ]);
+
+    const expectedPart = {
+      string: 'true,false,,,[object Object],,string,0',
+      headers: {
+        'content-disposition': 'form-data; name="array"',
+      },
+      fieldName: 'array',
+    };
+    expect(formData.getParts()[0]).toMatchObject(expectedPart);
+  });
 });