Fix bug causing context propagation in logs

Before this patch, the behavior was the following

```
Pliny.context(baz: "qux") do
end
Pliny.log(foo: "bar") # => "foo=bar baz=qux"
```

Now it is the logical

```
Pliny.context(baz: "qux") do
end
Pliny.log(foo: "bar") # => "foo=bar"
```