delete sessionID in requestHandler after closing the session - this fix will enable to request a new session with the same instance (after closing a running session before)
delete sessionID in requestHandler after closing the session
- this fix will enable to request a new session with the same instance (after closing a running session before)

refs #217

Risto-Stevcev/webdriverio,amitaibu/webdriverio,udnisap/webdriverio,silvenon/webdriverio,amitaibu/webdriverio,Pitzcarraldo/webdriverio,eyalzek/webdriverio,mike-suggitt/webdriverio,richardmcsong/webdriverio,Pitzcarraldo/webdriverio,saulshanabrook/webdriverio,mike-suggitt/webdriverio,stoeffel/webdriverio,eyalzek/webdriverio,noodle-learns-programming/webdriverio,cto/webdriverio,rossPatton/webdriverio,austek-mdsol/webdriverio,adjohnson916/webdriverio,vslinko-forks/webdriverio,austek-mdsol/webdriverio,silvenon/webdriverio,danielgil/webdriverio,monolithed/webdriverio,udnisap/webdriverio,sebastiendavid/webdriverio,wcainboundary/webdriverio,adjohnson916/webdriverio,cto/webdriverio,gabrielpeery/webdriverio,Mimiscita/webdriverio,drptbl/webdriverio,drptbl/webdriverio,sebastiendavid/webdriverio,matteonovelli/webdriverio,leahciMic/webdriverio,Risto-Stevcev/webdriverio,jhoguet/webdriverio,gabrielpeery/webdriverio,angelsanz/webdriverio,dandv/webdriverio,noodle-learns-programming/webdriverio,wcainboundary/webdriverio,rossPatton/webdriverio,milafrerichs/webdriverio,vslinko-forks/webdriverio,pazone/webdriverio,dandv/webdriverio,jhoguet/webdriverio,angelsanz/webdriverio,monolithed/webdriverio,leahciMic/webdriverio,richardmcsong/webdriverio,testingbot/webdriverjs,saulshanabrook/webdriverio,compulim/webdriverio,georgecrawford/webdriverio,danielgil/webdriverio,pazone/webdriverio,testingbot/webdriverjs,milafrerichs/webdriverio,Mimiscita/webdriverio,georgecrawford/webdriverio,matteonovelli/webdriverio,stoeffel/webdriverio,compulim/webdriverio