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.
Merge branch 'fix-empty-query-param'