Use `#send` in `.install_proxy_methods`

In order to proxy syntactic sugar methods (e.g. some_attribute=), use `#send`
in the module_eval.  Old approach would raise a syntax error in the for
`method_missing`/`.install_proxy_methods` for such assignment methods.
Minor: added #encoding: utf-8 to spec file
added info about send_only_modified_attributes