[BUG] Azure.Data.Tables SDK: Type error when accessing PartitionKey or RowKey in non-string property values are set.

[tq20202020]

Library name and version

Azure.Data.Tables 12.8.2

Describe the bug

var a = new TableEntity(partitionKey="partKey", rowKey="rowKey");
a["PartitionKey"] = new DateTime(); a["RowKey"] = new DateTime();
a.PartitionKey <--- throws type conversion error

Expected behavior

type error should be thrown when setting the value of PartitionKey/RowKey if we expect it to be string.
or accessing PartitionKey should return convered string value.

in addition, properties set through constructor should be immutable and should not be overwritten later by setting property value. so an exception should be raised when try to modify it in the property bag, this can avoid undefined behavior on which value should be respected (the one set in constructor, or the one set in property bag).

Actual behavior

var a = new TableEntity(partitionKey="partKey", rowKey="rowKey");
a["PartitionKey"] = new DateTime(); a["RowKey"] = new DateTime();
a.PartitionKey <--- throws type conversion error

Reproduction Steps

var a = new TableEntity(partitionKey="partKey", rowKey="rowKey");
a["PartitionKey"] = new DateTime(); a["RowKey"] = new DateTime();
a.PartitionKey <--- throws type conversion error

Environment

.NetCore