Combine two dictionaries of same type with custom operator.

Dictionary+PlusAssignment.swift

extension Dictionary {
    /// Combine two dictionaries of same type with `+` operator.
    ///
    /// - Important: When `left` and `right` are having a same key, the value from `right` will override
    ///              the value from `left`. E.g.
    ///              ```
    ///              var left = ["a": 1, "b": 2]
    ///              let right = ["a": 3, "c": 4]
    ///              left += right
    ///              print(left)
    ///              // ["a": 3, "b": 2, "c": 4,]
    ///              ```
    static func += (left: inout [Key: Value], right: [Key: Value]) {
        left.merge(right, uniquingKeysWith: { _, rightVal in rightVal })
    }
}