Incorrect cursor returned within resumption token.

Overview:

1) When running a harvester, after few requests the harvesting fails with the following error:

 Identical resumptionTokens
System.Exception: Identical resumptionTokens
 

 with following resumptionToken in response: 

 <resumptionToken cursor="500">...</resumptionToken>

2) The cursor never changes. It is required to populate cursor value properly according to OAI-PMH rules. As well need to enrich existing resumption token flow unit tests with cursor value verification. 
3) Srs may respond with 502 when requesting 500 records which leads to 500 error. It is required to re-request SRS when it responds with a 500+ status code instead of failing the oai-pmh request. Provide the unit tests for this as well.