Ion setHeader: Fix NPE in issue 201

https://github.com/koush/ion/issues/201#issuecomment-40509221
Merge branch 'master' of https://github.com/koush/ion

# By Koushik Dutta
# Via Koushik Dutta
* 'master' of https://github.com/koush/ion:
  Ion setHeader: Fix NPE in issue 201