fix simple trailing whitespace

fixes #742

https://bugs.ruby-lang.org/issues/7175

@bf4 @jeremy 
```
https://bugs.ruby-lang.org/issues/7175 trailing = is not standard compliant, but it's common in practice
```

This comment is inline with the code. The test is still "explained" by a bare URL about a Ruby behavior change. The linked URL gives a hint about why you made the PR. It doesn't really explain why we're stripping the equal sign.

1. The trailing equals is neither standard compliant nor non-compliant. It's ambiguous. This PR is making a behavior change to strip the equals and resolve the ambiguity. It was caused by Ruby changing to pass through the equals, causing a regression for us since we implicitly relied on its old behavior.

2. Trailing equals with no newline is not common in practice. Older versions of the mail lib stripped the body before decoding, causing this problem. It's much rarer to see a message actually arrive with malformed encoding.