[OSVDB#110796] Fix for Remote Code Execution weakness in FlavourSaver runtime.

Github user @nifoc (Daniel Kempens) discovered a remote code execution
flaw in FlavourSaver where the runtime was not sufficiently validating
methods being called on the context object, and using `send` to
violate method privacy barriers.

This allowed expressions such as `{{system "ls"}}` and
`{{eval "puts 1 + 1"}}` to be evaluated.

This fix involves guarding against calling private methods on the
template context by checking the `respond_to?` first and raising an
exception.
Merge branch 'master' of github.com:jamesotron/FlavourSaver