The user was able to vote as many investments as wanted in the first
heading voted. However in the second heading voted, only one investment
could be voted
This was due to the previous implementation, where you could only vote
in one heading. Note the `first` call in method
`heading_voted_by_user?(user)`
This commits simplifies the logic and allows voting for any investment
in any heading that the user has previously voted in
073cf74818