Fix issue #2212: Bson.get!BsonRegex does not compile
Fix issue #2212: Bson.get!BsonRegex does not compile

It was not possible to pass alias of m_data with const
qualifier into a function that mutates the reference.
Now passing full slice instead.

Also added unit test

chalucha/vibe.d,Geod24/vibe.d,gedaiu/vibe.d,etcimon/vibe.d,etcimon/vibe.d,rejectedsoftware/vibe.d