[#38] Add instance default value

[lwitkowski]

For explanation 'why' refer to #38

Changes in this PR from end user point of view: NotFoundException thrown from /country/pl:

Before:

{
  "title": "Not Found",
  "status": 404,
  "detail": "Country not found"
}

After:

{
  "title": "Not Found",
  "status": 404,
  "detail": "Country not found",
  "instance": "/country/pl"
}

What is important is that adding default values happens after problem is logged to console. This means type and instance fields are not logged, otherwise it could lead to personal info (like ssn sent as path param) being logged, which is usually not a good idea.

Closes #38

[pazkooda]

Approved (with one comment - could be fixed later).

[pazkooda]

Quite not-self-contained test :(.
Why I need to go outside of this class to understand why path has /endpoint?

[lwitkowski]

True that, will address that in separate PR, where I already have made some changes to this class.

[github-actions]

Unit Test Results

15 files  ±0  15 suites  ±0   22s ⏱️ ±0s
47 tests ±0  47 ✔️ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 07c9347. ± Comparison against base commit 07c9347.