avoid buggy collapse_adjacent_encoding()

avoiding collapsing might be inefficient bit, but it works anyway in
cases like:

```
  =?UTF-8?B?56eB44Gu5ZCN5YmN44Gv5p2+5pys44Gn44GZ44CC?=
  =?UTF-8?B?UnVieeOCkumWi+eZuuOBl+OBpuOBhOOBvuOBmQ==?=
```

that will be broken by collapse_adjacent_encoding().