Release 4.0.4: Flutter web performance improvements

update doc

# 4.0.2

- Add QueuedInterceptor
- merge cfug#1316 cfug#1317

Add QueuedInterceptor

# 4.0.1

- merge pr cfug#1177 cfug#1196 cfug#1205 cfug#1224 cfug#1225 cfug#1227 cfug#1256 cfug#1263 cfug#1291
- fix cfug#1257

update doc

1. fix cfug#877 'dio.interceptors.errorLock.lock()'

2. fix cfug#851
3. fix cfug#641

add test cases

Merge pull request cfug#639 from Endebert/patch-1

fix(transformer): reduce memory consumption

Merge pull request cfug#596 from HackrHIVE/patch-1

minor change in example (FormData)