Add autocorrect for Lint/AssignmentInCondition

[r7kamura]

I think adding support for unsafe autocorrection to Lint/AssignmentInCondition would be more useful in some cases. Now that SafeAutoCorrect: false allows us to indicate to users that it is unsafe, I don't think we will have much trouble because of this support.

I decided to replace the code as assignment by default in this autocorrection because it does not change the behavior of the current code. If there seem to be many use cases where the behavior of autocorrection needs to be changed, we can add such configuration in this Pull Request or the future change. (In fact, is there any Cop that allows the autocorrection behavior to be switched by configuration in that way?)

Depending on the policy, this type of autocorrection support may not be acceptable in RuboCop. In which case, please do not hesitate to close this Pull Request.

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[r7kamura]

As background, I had to migrate a bit large old codebase from rubocop 0.44 to 1.39. I thought it would be easy to complete with autocorrection because the tests were well written, but this cop did not support autocorrection, so I had to manually rewrite a lot of such ambiguous assignment code, which was very complicated task.
In other words, I am saying that there are cases where it is useful enough to have it provided, even if it is not a perfect autocorrection.

[koic]

I think any configuration is not required for this autocorrection. So this unsafe autocorrection approach would be better.

[bbatsov]

I like the proposal. Thanks!