swap out broken nil checks (not harming anything, but not working as evidenced by not getting hit by any test cases)

Sources/JSONAPI/Document/ResourceBody.swift

@@ -71,10 +71,12 @@ extension SingleResourceBody {
 	public func encode(to encoder: Encoder) throws {
 		var container = encoder.singleValueContainer()
 
-		if (value as Any?) == nil {
-			try container.encodeNil()
-			return
-		}
+        let anyNil: Any? = nil
+        let nilValue = anyNil as? Entity
+        guard value != nilValue else {
+            try container.encodeNil()
+            return
+        }
 
 		try container.encode(value)
 	}

Sources/JSONAPI/Resource/Relationship.swift

@@ -190,10 +190,6 @@ extension ToOneRelationship: Codable where Identifiable.Identifier: OptionalId {
 	public func encode(to encoder: Encoder) throws {
 		var container = encoder.container(keyedBy: ResourceLinkageCodingKeys.self)
 
-		if (id as Any?) == nil {
-			try container.encodeNil(forKey: .data)
-		}
-
 		if MetaType.self != NoMetadata.self {
 			try container.encode(meta, forKey: .meta)
 		}

Tests/JSONAPITests/ResourceBody/ResourceBodyTests.swift

@@ -100,6 +100,9 @@ class ResourceBodyTests: XCTestCase {
 		XCTAssertEqual(combined.values.count, 3)
 		XCTAssertEqual(combined.values, body1.values + body2.values)
 	}
+}
+
+extension ResourceBodyTests {
 
 	enum ArticleType: ResourceObjectDescription {
 		public static var jsonType: String { return "articles" }