Fix Issue 255's Strict Json Transformer

[JoaquinIglesiasTurina]

The method for walk :or from spec-tools core was changed.

Previously it was reducing over all the spec's items (different specs within the spec/or).
It was essentialy coercing to a given item and the next given item and so forth and so on.
This caused an strict-json-transformer to behave inconsistently.

@ikitommi 's comment at issue 178, is implemented here.
The method now coerces to all items, and chooses one based on validity or previous behavior.

Issues with the proposed solution

The main issue is that the behavior of the method changes. As it only applies a particular coercion now.

Testing Composed
In that particular test, the value under keys ["keys" ::c2] gets coerced to a keyword.
That is not in thespec.

On the same example ["keys" :c1] will not get coerced to an int now. But that is how coerce works
as ilustrated here.

Parametrization Matters
Minimal examples have been given for this particular issue. Those have been adapted as tests.
However, the spec must be written in a particular way. I find this a major flaw.
This is the clearest example.