Symbol encoding changes after defining a method with same name

### Environment

Provide at least:
* JRuby-9.1.8.0
* OSX, Fedora...

### Expected Behavior

```
a = :something
a.encoding # #<Encoding:US-ASCII>
def something; end
a.encoding # should be #<Encoding:US-ASCII>
```

### Actual Behavior

```
a = :something
a.encoding # #<Encoding:US-ASCII>
def something; end
a.encoding # it is now #<Encoding:ASCII-8BIT>
```