Fix Context.{post,request} to not behave oddly when 'headers' parameter is omitted.
Fix Context.{post,request} to not behave oddly when 'headers' parameter is omitted.

sullivanmatt/splunk-sdk-python,splunk/splunk-sdk-python,lowtalker/splunk-sdk-python,ww9rivers/splunk-sdk-python,splunk/splunk-sdk-python,kkirsche/splunk-sdk-python,kkirsche/splunk-sdk-python,kkirsche/splunk-sdk-python,kkirsche/splunk-sdk-python