UTF-8 strictness is Event field setter

This is a followup on #5349

I am a bit perplexed on the encoding validation upon setting an Event field string value with non UTF-8 encoding. I wrote the following spec

``` ruby
it "should fail on non UTF-8 encoding" do
e = LogStash::Event.new
s1 = "\xE0 Montr\xE9al".force_encoding("ISO-8859-1")
expect(s1.encoding.name).to eq("ISO-8859-1")
expect(s1.valid_encoding?).to eq(true)
e.set("test", s1)
s2 = e.get("test")

# it currently fails on this line below
expect(s2.encoding.name).to eq("UTF-8")
expect(s2.valid_encoding?).to eq(true)
end
```

This tests fails a the `expect(s2.encoding.name).to eq("UTF-8")` line with

```
1) LogStash::Event []= should fail on non UTF-8 encoding
Failure/Error: expect(s2.encoding.name).to eq("UTF-8")

expected: "UTF-8"
got: "ISO-8859-1"
```

Should we enforce string UTF-8 encoding at the Event field setter? It could be done in the Event class or in the BiValue transformations.

Thoughts?