Fix address with Q-encoded display name containing double quotes

I added a spec for parsing the following address:

```
=?ISO-8859-1?Q?"Jan_Kr=FCtisch"?= <jan@krutisch.de>
```

Before the fix, display name was parsed as `Jan_Kr=FCtisch` because when a quoted string was found (ie. `"Jan_Kr=FCtisch"`), it was used as the display name and the surrounding characters were ignored.

After the fix, display name is properly encoded as `Jan Krütisch`. Code-wise, the `qstr` variable is now used as the display name instead of `phrase` only if `qstr` is equal to `phrase` minus the quotes.

I would have liked to avoid creating the `%("#{qstr}")` string, but I did not see another way. Other than that performance/memory should not be affected.