Fixing strip-extra-keys-transformer for recursive map encoding

[tomasd]

This PR fixes an issue when strip-extra-keys-transformer was used to encode the :map-of used in a recursive way. Transformer needs to run before the encoder otherwise it sees already encoded value which does not pass the validation.

There is a pending issue when an invalid key in :map-of instead of being removed from the value invalidates the whole :map-of. This would probably require recursive validation, which may add some computational complexity.

[ikitommi]

There is a pending issue when an invalid key in :map-of instead of being removed from the value invalidates the whole :map-of. This would probably require recursive validation, which may add some computational complexity.

a pending issue? but, merging this, thanks!