ditch centos which had non-test-related error. fix type mismatch heuristic.

Author: mattpolzin

.github/workflows/tests.yml

@@ -35,7 +35,6 @@ jobs:
           - swift:5.5-amazonlinux2
           - swift:5.6-bionic
           - swift:5.6-focal
-          - swift:5.6-centos7
           - swift:5.6-amazonlinux2
     container: ${{ matrix.image }}
     steps:

Sources/JSONAPI/Document/DocumentDecodingError.swift

@@ -33,7 +33,9 @@ public enum DocumentDecodingError: Swift.Error, Equatable {
             self = .primaryResourceMissing
         case .valueNotFound(let type, let context) where Location(context) == .data && type == UnkeyedDecodingContainer.self:
             self = .primaryResourcesMissing
-        case .typeMismatch(_, let context) where context.debugDescription.hasSuffix("but found null instead."):
+        case .typeMismatch(let type, let context) where Location(context) == .data && type is AbstractResourceObject.Type && context.debugDescription.hasSuffix("but found null instead."):
+            self = .primaryResourceMissing
+        case .typeMismatch(let type, let context) where Location(context) == .data && type == UnkeyedDecodingContainer.self && context.debugDescription.hasSuffix("but found null instead."):
             self = .primaryResourcesMissing
         default:
             return nil