feat: add setBoundary method

Readme.md

@@ -189,6 +189,7 @@ form.submit({
 - [_Void_ append( **String** _field_, **Mixed** _value_ [, **Mixed** _options_] )](https://github.com/form-data/form-data#void-append-string-field-mixed-value--mixed-options-).
 - [_Headers_ getHeaders( [**Headers** _userHeaders_] )](https://github.com/form-data/form-data#array-getheaders-array-userheaders-)
 - [_String_ getBoundary()](https://github.com/form-data/form-data#string-getboundary)
+- [_Void_ setBoundary()](https://github.com/form-data/form-data#void-setboundary)
 - [_Buffer_ getBuffer()](https://github.com/form-data/form-data#buffer-getbuffer)
 - [_Integer_ getLengthSync()](https://github.com/form-data/form-data#integer-getlengthsync)
 - [_Integer_ getLength( **function** _callback_ )](https://github.com/form-data/form-data#integer-getlength-function-callback-)
@@ -220,11 +221,15 @@ form.append( 'my_file', fs.createReadStream('/foo/bar.jpg'), {filename: 'bar.jpg
 This method adds the correct `content-type` header to the provided array of `userHeaders`.  
 
 #### _String_ getBoundary()
-Return the boundary of the formData. A boundary consists of 26 `-` followed by 24 numbers
+Return the boundary of the formData. By default, the boundary consists of 26 `-` followed by 24 numbers
 for example:
 ```javascript
 --------------------------515890814546601021194782
 ```
+
+#### _Void_ setBoundary(String _boundary_)
+Set the boundary string, overriding the default behavior described above.
+
 _Note: The boundary must be unique and may not appear in the data._
 
 #### _Buffer_ getBuffer()

index.d.ts

@@ -36,6 +36,7 @@ declare class FormData extends stream.Readable {
     callback?: (error: Error | null, response: http.IncomingMessage) => void
   ): http.ClientRequest;
   getBuffer(): Buffer;
+  setBoundary(boundary: string): void;
   getBoundary(): string;
   getLength(callback: (err: Error | null, length: number) => void): void;
   getLengthSync(): number;

lib/form_data.js

@@ -305,6 +305,10 @@ FormData.prototype.getHeaders = function(userHeaders) {
   return formHeaders;
 };
 
+FormData.prototype.setBoundary = function(boundary) {
+  this._boundary = boundary;
+};
+
 FormData.prototype.getBoundary = function() {
   if (!this._boundary) {
     this._generateBoundary();

test/integration/test-set-boundary.js

@@ -0,0 +1,23 @@
+var common = require('../common');
+var assert = common.assert;
+
+var FormData = require(common.dir.lib + '/form_data');
+
+(function testSetBoundary() {
+  var userBoundary = '---something';
+  var form = new FormData();
+  form.setBoundary(userBoundary);
+
+  assert.equal(form.getBoundary(), userBoundary);
+})();
+
+(function testUniqueBoundaryPerFormAfterSet() {
+  var userBoundary = '---something';
+  var formA = new FormData();
+  formA.setBoundary(userBoundary);
+
+  var formB = new FormData();
+
+  assert.equal(formA.getBoundary(), userBoundary);
+  assert.notEqual(formA.getBoundary(), formB.getBoundary());
+})();