FormFilter does not adjust Content-Length header

[pputerla]

Describe the bug FormFilter can recreate produce body bigger than original but Content-Length header is not changed. Any other processing may fail because it may read body up to Content-Length trimming it in the process.

Sample Example is exlamation mark character.

When sending x-www-form-urlencoded form a browser it will not encode "!" into %21. It will count this character as 1 byte

However FormFilter will encode the character here increasing the output by 2 bytes