Cloud Storage - character = encoded to %3D on bucket key name

[archenroot]

So i am encoding to bucket key the object metadata like this:
accountantBusinessName=Ing. Monika Frydrychová,accountantBusinessId=86556231/clientBusinessName=Ladislav Zítka,clientBusinessId=74819071/incoming/documentId=fec70948-6a33-4ff5-bb27-ada7f834b283.jpg

But when written in google cloud storage some of characters are encoded which is bad behavior:
accountantBusinessName%3DIng. Monika Frydrychová,accountantBusinessId%3D86556231/clientBusinessName%3DLadislav Zítka,clientBusinessId%3D74819071/incoming/documentId%3Dfec70948-6a33-4ff5-bb27-ada7f834b283.jpg

Please not that spaces neither accent characters are not encoded, only '=' char.

Also I have tested for example with Java implementation of google libs and it works good. I suspect something in this dart lib.

[archenroot]

any idea how to solve this?