Fix running tests for core clr
Fix running tests for core clr

Wrap System.Net.Http using directive in preprocessor directive.

elastic/elasticsearch-net,adam-mccoy/elasticsearch-net,CSGOpenSource/elasticsearch-net,elastic/elasticsearch-net,adam-mccoy/elasticsearch-net,CSGOpenSource/elasticsearch-net,CSGOpenSource/elasticsearch-net,adam-mccoy/elasticsearch-net