Merge dictionaries in place as described in http://ericasadun.com/2015/07/08/swift-merging-dictionaries/

extension.Dictionary.swift

extension Dictionary {
    mutating func unionInPlace(dictionary: Dictionary<Key, Value>) {
        for (key, value) in dictionary {
            self[key] = value
        }
    }
    mutating func unionInPlace<S: SequenceType where S.Generator.Element == (Key, Value)>(sequence: S) {
        for (key, value) in sequence {
            self[key] = value
        }
    }
}