Fixes intermittent runtime exceptions in OptionDecode.

OptionDecode was using WeakTypeTag from the new scala refelction API. In 2.10 this is not thread safe and which can cause runtime exceptions. Changed the method to use Manifest instead.

This issues is tracked by Scala as https://issues.scala-lang.org/browse/SI-6240 and is apparently fixed in 2.11.

@benl23x5 @SamRoberts this should fix the intermittent errors you saw.
is this line meant to be split?