fix simple trailing whitespace

fixes #742

https://bugs.ruby-lang.org/issues/7175

@bf4 @jeremy 
The bug is not closed/fixed, but "Third Party's Issue", and for incoming email that means it's my issue since I cannot tell customers that they should use "standards compliant encoders", they only got what they got and don't want to see trailing equals. As noted in the ruby issue "But in practice I'm seeing that it's common."

I know it's not the "100% right thing" to do, but I only see 3 solutions: on by default default, optional/configureable, it stays my monkeypatch.