remove unnecessary attr_writer

because we already have attr_accessor
Spec for issue #865.
Fixed declared(params, include_missing: false) now returning attributes with nil and false values.
Spec from #857, confirm fix for included helper modules no longer available in nested mounted resources.
Specs confirming fixes for #861, #862.
Merge pull request #869 from yesmeck/patch-0

remove unnecessary attr_writer
Merge pull request #870 from yesmeck/master

make travis green