Fix/bug 1691 preserving change language binding

[timheilman]

Checklist

• only relevant code is changed (make a diff before you submit the PR)
• run tests npm run test
• tests are included
• commit message and code follows the Developer's Certification of Origin

Checklist (for documentation change)

• only relevant documentation part is changed (make a diff before you submit the PR)
• motivation/reason is provided
• commit message and code follows the Developer's Certification of Origin

[coveralls]

coverage: 97.029% (+0.04%) from 96.986%
when pulling 3c3f512 on timheilman:fix/bug-1691-preserving-changeLanguage-binding
into ac3f71c on i18next:master.

[timheilman]

Sorry about the initial bad fix in #1716 . A dev issue was uncovered over at #1716 (comment) . Unfortunately I still cannot figure out how to test the bug 1691 proper in a unit test, but the dev issue does have a test added that:

1. succeeds before Fix/bug 1691 make returned t function identical upon second effect run in strict mode #1716
2. fails after Fix/bug 1691 make returned t function identical upon second effect run in strict mode #1716 , demonstrating the dev issue
3. succeeds after this PR

I also have tested this PR manually for both bug 1691 proper as well as for the dev issue. I welcome @adrai and @medihack 's eyes on this second attempt, as well as anyone else's interested. No time pressure.

[adrai]

thank you... it's included in v14.0.5

[adrai]

fyi: verified to work also in this example 8334298 👍