Fix GCDWebServerParseURLEncodedForm to allow empty values.
Fix GCDWebServerParseURLEncodedForm to allow empty values.

If the input string is something like foo=&bar= then both foo and bar
should be in the result, with the empty string as their corresponding value.

The [scanner scanUpToString:@"&" ...] call was returning failure, because
it was already positioned at the &.  In this situation, just set value to the
empty string.

TimOliver/GCDWebServer,MinorUncle/GCDWebServer,liuchungui/GCDWebServer,sagmor/GCDWebServer,donpark/GCDWebServer,swisspol/SANDBOX,pollarm/GCDWebServer,silvansky/GCDWebServer,palmin/GCDWebServer,8fit/GCDWebServer,pollarm/GCDWebServer,sagmor/GCDWebServer,donpark/GCDWebServer,CrossWaterBridge/GCDWebServer,cnbin/GCDWebServer,TimOliver/GCDWebServer,tipbit/GCDWebServer,hushifei/GCDWebServer,zh-XN/GCDWebServer,edmstudio/GCDWebServer,boherna/GCDWebServer,zh-XN/GCDWebServer,zh-XN/GCDWebServer,Coppertino/GCDWebServer,sagmor/GCDWebServer,swisspol/SANDBOX,CrossWaterBridge/GCDWebServer,1yvT0s/GCDWebServer,aoman89757/GCDWebServer,palmin/GCDWebServer,silvansky/GCDWebServer,aoman89757/GCDWebServer,silvansky/GCDWebServer,fluffyemily/GCDWebServer,swisspol/SANDBOX,1yvT0s/GCDWebServer,hushifei/GCDWebServer,liuchungui/GCDWebServer,MinorUncle/GCDWebServer,cnbin/GCDWebServer,palmin/GCDWebServer,ddnbgroup/GCDWebServer,tipbit/GCDWebServer,tipbit/GCDWebServer,sagmor/GCDWebServer,fluffyemily/GCDWebServer,zh-XN/GCDWebServer,palmin/GCDWebServer,tipbit/GCDWebServer,TimOliver/GCDWebServer,TimOliver/GCDWebServer,donpark/GCDWebServer,cnbin/GCDWebServer,liuchungui/GCDWebServer,Coppertino/GCDWebServer,8fit/GCDWebServer,ddnbgroup/GCDWebServer,liuchungui/GCDWebServer,edmstudio/GCDWebServer,boherna/GCDWebServer,edmstudio/GCDWebServer,MinorUncle/GCDWebServer,boherna/GCDWebServer,ddnbgroup/GCDWebServer,Coppertino/GCDWebServer,1yvT0s/GCDWebServer,silvansky/GCDWebServer,fluffyemily/GCDWebServer,hushifei/GCDWebServer,MinorUncle/GCDWebServer,ddnbgroup/GCDWebServer,cnbin/GCDWebServer,Coppertino/GCDWebServer,1yvT0s/GCDWebServer,aoman89757/GCDWebServer,pollarm/GCDWebServer,CrossWaterBridge/GCDWebServer,fluffyemily/GCDWebServer,swisspol/SANDBOX,pollarm/GCDWebServer,boherna/GCDWebServer,8fit/GCDWebServer,edmstudio/GCDWebServer,donpark/GCDWebServer,CrossWaterBridge/GCDWebServer,8fit/GCDWebServer,hushifei/GCDWebServer,aoman89757/GCDWebServer