FormBuilder#has_many does not support multiple inputs

I was actually using it and it makes sense to do so. By bisecting I've found the commit where it was introduced (#3486):
```
6afaf58357f9606835b3d3b17356a85abcb54cae is the first bad commit
commit 6afaf58357f9606835b3d3b17356a85abcb54cae
Author: Piers Chambers <piers@varyonic.com>
Date:   Sun Sep 21 20:12:40 2014 -0400

    Refactor FormBuilder, replacing form_buffers with Arbre components.  See PR #3486 for discussion.

:040000 040000 dfe2c2b07bd7269df5bf153e0dd14441704b1be8 61297dea084b349b70c13e5f9782166a7cbe9285 M	lib
:040000 040000 a3852ad0ef3ad4e6c8f88c3f9b1d1431cf61a833 1e0c445a3193c923b9ce0fbf4ee0a16b9dc95add M	spec
```
I was using https://gist.github.com/mikz/c00d57f879221b949411 as the test file.

Also this PR has the failing test.

Ideas how to fix it? /cc @varyonic