Check for overridden initializer values

closes #153

Treat models and arrays of models as safe input to redirect_to

fixes #143

instead of just an empty string. This should happen almost never, but
hopefully this will prompt people to report bugs if it does happen
instead of hiding them.

Remove special Sexp nodes from HAML processor

Fix uses of `method_missing` for Sexp

This is an enhancement and and a bug fix.  Models use polymorphic routes and should be considered safe as do arrays.  Treat arrays containing things other than models as unsafe as well.

There should never be the case where the target of a :call Sexp is a
symbol, it should always be either `nil` or a Sexp. Brakeman should not
be building invalid Sexps.

for the very few times the original Sexp `method_missing`
behavior is expected

Surprised this never caused any errors

Brakeman was generating invalid call Sexps with `:_hamlout`
as the target for `_hamlout.options` and `hamlout.buffer`.
However, these special Sexps were not really be handled in Brakeman
anywhere, and there was no real reason to make special Sexps for them.

If exceptions occured while formatting code,
OutputProcessor#format would return `nil`. Now it should return
a string instead so it can be used safely.

Conflicts:
	test/tests/test_brakeman.rb

because it was checking the options for the wrong value