Fix :: avoid mutating Field default value

[PrettyWood]

Change Summary

#1210 introduced a regression by mutating default value of a field.

Related issue number

fix #1412

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)

[codecov]

Codecov Report

Merging #1413 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1413   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         3741      3741           
  Branches       740       740           
=========================================
  Hits          3741      3741           

Impacted Files	Coverage Δ
pydantic/fields.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1f071f1...b0f0821. Read the comment docs.

[samuelcolvin]

Looks good to me.

I guess we might need a patch release for this?

[PrettyWood]

I was going to say a 1.5.1 would most likely be needed yes. Sorry about that

[samuelcolvin]

Okay, but I hate projects that make 3 releases in a day, each with one tiny fix.

Let's wait at least for the rest of today (UK time) to see if anything else comes up.

Then I'll deploy. Make sense?

[PrettyWood]

Agreed