Fix batch failed with relative url & Content-Transfer-Encoding have duplicate binary

[sleidor]

Exception with relative URLs

• Add check for IsAbsoluteUri during HttpMessageContent;

Duplicate binary in Content-Transfer-Encoding

• Remove duplicate headers in PanoramicData.OData.Client/ODataClient.Batch.cs, as constructor HttpMessageContent already init with that headers;

UnitTests

• mark datetime UTC for Read_SimpleDateFormat_ParsesCorrectly(), it will failed if your local timezone is UTC + number;
• add WriteToStreamAsync_ShouldIncludeRequiredHeaders() for checking headers in batch;