Added ActionController::Parameters.each_value methods

[lzap]

Although Parameters class is far from implementing Hash and it's obvious that
it's not primarly goal to achieve it, having the two convinient methods for
iterating through parameters makes it more quacking like Hash:

• each_value
• each_key

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[lzap]

I see a left over. To be removed.

[eileencodes]

I'm confused. Do you mean you need to add this back or this was left over in a previous PR? It looks like from the example below that the keys are strings.

[lzap]

Sorry I mean, I am going to remove this hunk in the next rebase. Let's do this.

[rafaelfranca]

We don't want to make Parameters support all Hash values. I'm fine with each keys but each values will skip the parameters protection that we want to enforce. If you want to use those method please transform the parameters in hash using to_h.

[rafaelfranca]

Although we already have to_pair. Let's add those methods but delegate each_pair and each_value to to_h. If you are calling those methods in a un-permitted Parameter instance we should raise.

[rafaelfranca]

Forget about that, changing behaviour of each to yield hashes will be a breaking change. each_key is already supported. Let's change this PR to only implement each_value.

[lzap]

Rebased: new implementation of only each_value, tests for both each_value and each_key. For the latter I am asserting String type, not sure what else I can assert.