Fix osx function: "not valid in this context"

I have been continually having problems with the "trash" function provided by the osx plugin, getting this error:

    trash:local:2: not valid in this context:

It seems like adding quotes around the assignment in [osx.plugin.zsh#L142](https://github.com/robbyrussell/oh-my-zsh/blob/master/plugins/osx/osx.plugin.zsh#L142) fixed my problem.

Does this look good to you? Thanks!
Merge pull request #2974 from elliottwilliams/patch-1

Fix osx function: "not valid in this context"