fix: resolve MySQL unique index check when bigNumberStrings is false

[SteveByerly]

Part of fix for #2737 and #2943

[SteveByerly]

Further information:
MySQL indexes have a Non_unique property that normally has a value of either "0" or "1".

When the option for bigNumberStrings is set to false (where big numbers are not coerced into strings), the value for Non_unique comes back as the numeric 0 or 1.

There may be other locations affected by this subtle difference, but I have only encountered it when dealing with migrations. Every new migration attempts to update those columns with a unique index (including dropping and re-adding the index) even though the column definition has not changed.

There is at least one other case that causes unchanged columns to be updated in every migration, but it has a workaround and I would prefer fixing it in a separate PR.

Issue:

• Numeric columns without a width specified are always recreated
  Source:
• MysqlDriver.prototype.findChangedColumns compares the width defined in the decorator with the value returned from the database. If no width is provided, the value is undefined and compared to the default value from the DB.

[fw623]

I found the same problem with unique Index getting dropped and readded every migration.
My attempt at fixing: PR4819
This should also fix Issue #3841

[pleerock]

imagine somebody reading this code. Can you please explicitly specify what logic should be in there (as I understood we should check for === "0" && === 0? If so, please change it

[SteveByerly]

@pleerock updated the code to be more explicit.

Pulled in latest from master and rebased.

I debated pulling that logic out into a method on the QueryRunner, but didn't want to overstep and make assumptions.
something like:
private isUniqueIndex(constraint): boolean

If you'd rather go that route, I can update. Otherwise let me know if there's anything else you'd like changed.

[pleerock]

@fw623 correct its a duplicate, but here we also have a test and both drivers covered. We'll need to close your PR once this one is completed

[pleerock]

I think it looks okay now, thanks