Fixed isFormData predicate; (#4413)

Added support for automatic object serialization to FormData if `Content-Type` is `multipart/form-data`;
Added support for FormData to be overloaded using `config.env.FormData` option;
Added support for FormData in node.js environment through `form-data` package;

index.d.ts

@@ -107,6 +107,9 @@ export interface AxiosRequestConfig<D = any> {
   transitional?: TransitionalOptions;
   signal?: AbortSignal;
   insecureHTTPParser?: boolean;
+  env?: {
+    FormData?: new (...args: any[]) => object;
+  };
 }
 
 export interface HeadersDefaults {