Merge branch 'hotfix/45' into develop

Forward port #45
Merge branch 'hotfix/47' into develop

Forward port #47
[#43] Allow arbitrary fields in ApiProblem

- Specification allows adding arbitrary additional parameters in the
  result. Clients should not rely on this information, but it can still
  provide information that aids in understanding the error returned.
- Written to ensure required fields are never overwritten by additional
  details.
Merge branch 'feature/43-api-problem-extensions' into develop

Fixes #43
Merge branch 'develop'

Closes #43