Make TopLevelEncoder implementation overridable

[kkebo]

TopLevelEncoder implementation was added by #175. However, its method cannot be overridden. If it can be, we can use custom root key, root attributes, or header even when we use Combine-style like this:

final class MyEncoder: XMLEncoder {
    override func encode<T>(_ value: T) throws -> Data where T : Encodable {
        try self.encode(value, withRootKey: "foo", rootAttributes: nil, header: XMLHeader(version: 1.0, encoding: "UTF-8"))
    }
}

So, I proposed the solution.

[codecov[bot]]

Codecov Report

Merging #182 into master will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #182   +/-   ##
=======================================
  Coverage   73.32%   73.32%           
=======================================
  Files          43       43           
  Lines        2332     2332           
=======================================
  Hits         1710     1710           
  Misses        622      622           

Impacted Files	Coverage Δ
Sources/XMLCoder/Decoder/XMLDecoder.swift	76.29% <ø> (-0.35%)	:arrow_down:
Sources/XMLCoder/Encoder/XMLEncoder.swift	85.61% <100.00%> (+0.21%)	:arrow_up:

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[kkebo]

@MaxDesiatov It seems that danger-lint has failed in XMLDecoder, but I didn't change XMLDecoder at all. Do you have any ideas?

[MaxDesiatov]

No worries, that seems like a SwiftLint bug 🙂