[jwe] Add option to explicitly clear per-recipient headers ("header") for flattened JSON serialization

[lestrrat]

fixes #1470

After much going back and forth, I have nailed down that the ONLY time the current implementation creates duplicate entries (that matter) are when flattened JSON serialization is used.

For full JSON serialization, we didn't merge anything to start with. This was fine. It's the user's responsibility to figure out how to create headers that are completely disjoint. But for flattened JSON, we merged the per-recipient headers and the protected header, and yet kept the per-recipient headers in tact. The fix is to nuke the headers stored in the per-recipient headers after merging -- the merge is NOT necessary, but it is my understanding that we would be erring on the safer side if we merged everything into the protected header so that the entirety of this metadata is used as AAD, thus making the flattened JSON serialization that we produce analogous to the compact format.

Now, I'm sure there is code somewhere that already relies on this behavior, so instead of just "fixing" this, we introduced an option to explicitly enable the new behavior. Code that wants to generate a flattened JSON serialization that clears the "header" value should call jwe.Encrypt() with jwe.WithLegacyHeaderMerging(false).

[ilya-korotya]

Now everything works as expected 🎉