Fix circular dependency detection

[cbaker6]

Fixed a bug where the ParseEncoder incorrectly detects a circular dependency when two child objects are the same. The failing test case is below:

func testAllowFieldsWithSameObject() throws {
        var score = GameScore(score: 10)
        var level = Level()
        level.objectId = "nice"
        score.level = level
        score.nextLevel = level

        score.ensureDeepSave { (_, _, parseError) in
            XCTAssertNil(parseError)
        }
    }

[codecov]

Codecov Report

Merging #194 (efd24b7) into main (85b3e76) will increase coverage by 0.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #194      +/-   ##
==========================================
+ Coverage   82.88%   83.02%   +0.14%     
==========================================
  Files          73       73              
  Lines        6911     6912       +1     
==========================================
+ Hits         5728     5739      +11     
+ Misses       1183     1173      -10     

Impacted Files	Coverage Δ
Sources/ParseSwift/Coding/ParseEncoder.swift	75.31% <100.00%> (+0.15%)	⬆️
Sources/ParseSwift/Protocols/Objectable.swift	96.15% <100.00%> (+12.82%)	⬆️
Sources/ParseSwift/Types/Pointer.swift	83.33% <100.00%> (+2.48%)	⬆️
Sources/ParseSwift/Objects/ParseUser.swift	81.53% <0.00%> (-0.16%)	⬇️
Sources/ParseSwift/Objects/ParseObject.swift	78.72% <0.00%> (+1.32%)	⬆️
Sources/ParseSwift/Coding/ParseCoding.swift	91.48% <0.00%> (+2.12%)	⬆️

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