Try out the following which did work for me.
Method 1:
Tested on AT&T Fusion working fine.
Method 2:
Use the method given by shkschneider
adb shell am start -a android.intent.action.MAIN -n com.android.settings/.wifi.WifiSettings
adb shell input keyevent 20 & adb shell input keyevent 23
This works only if you have USB debugging turned on. If you do not have USB debugging turned on under Development do the following.
Tested on AT&T fusion working fine.