Sam Saffron 0a5a6dfded DEV: stop mutating inputs as a side effect
We had quite a few cases in core where inputs are being mutated as a side
effect of calling a method.

This handles all the cases where specs caught this.

Mutating inputs makes code harder to reason about. Eg:

```
frog = "frog"
jump(frog)
puts frog
"fly" # ?????
```

This commit is part of a followup commit that adds # frozen_string_literal
to all our specs.
2019-04-30 10:25:53 +10:00
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