graft of upstream fix https://github.com/mono/mono/pull/9342/files for case 1070667

Author: Michael DeRoy

mcs/class/System.Runtime.Serialization/ReferenceSources/JsonFormatReaderGenerator_static.cs

@@ -508,7 +508,9 @@ void ReadSimpleDictionary (CollectionDataContract collectionContract, Type keyVa
 					var jsonMemberName = XmlObjectSerializerReadContextComplexJson.GetJsonMemberName (xmlReader);
 					object key = null;
 
-					if (keyParseMode == KeyParseMode.UsingParseEnum)
+					if (keyParseMode == KeyParseMode.AsString)
+						key = jsonMemberName;
+					else if (keyParseMode == KeyParseMode.UsingParseEnum)
 						key = Enum.Parse (keyType, jsonMemberName);
 					else if (keyParseMode == KeyParseMode.UsingCustomParse)
 						key = keyDataContract.ParseMethod.Invoke (null, new object [] {jsonMemberName});