Squashed 'subtree/bokehjs/' changes from ae4fc78..b939d76

b939d76 instead of checking for png as empty string, just using false eval (python changes cause a different value to be sent)
aef8524 fixed typo (missing indent)

git-subtree-dir: subtree/bokehjs
git-subtree-split: b939d76e65490ceb1a598b9cb5eb9565a8442b29
Merge commit '163821c1d0624def0deae5a86e981fbbec4d120a' into feature/updatebokehjs