Render leftButtonImage (#838)
Render leftButtonImage (#838)

* Fix for leftButtonIcon not rendering

* check for buttonImage instead of menuIcon

* tidied up the setting of Image

peteychuk/react-native-router-flux,mcabs3/react-native-router-flux,Wondermall/react-native-router-flux,mcabs3/react-native-router-flux,Wondermall/react-native-router-flux,charpeni/react-native-router-flux,Swordsman-Inaction/react-native-router-flux,hungtn/react-native-router-flux,peteychuk/react-native-router-flux,gijoehosaphat/react-native-router-flux,dmazin/react-native-router-flux,dmazin/react-native-router-flux,dmazin/react-native-router-flux,gectorat/react-native-router-flux,gectorat/react-native-router-flux,charpeni/react-native-router-flux,mcabs3/react-native-router-flux,gectorat/react-native-router-flux,peteychuk/react-native-router-flux,gijoehosaphat/react-native-router-flux,Swordsman-Inaction/react-native-router-flux,charpeni/react-native-router-flux,Swordsman-Inaction/react-native-router-flux,hungtn/react-native-router-flux,gijoehosaphat/react-native-router-flux,Wondermall/react-native-router-flux,aksonov/react-native-router-flux