Update tag_helper.rb

Allow string value to take precedent for BOOLEAN_ATTRIBUTES in tag_options.

This specifically allows overrides for boolean HTML tags that have behaivior based on inclusion as well as value such as the `async` attribute of the `<script>` tag in Firefox https://developer.mozilla.org/en-US/docs/Web/HTML/Element/script

This specifically works around an issue in Firefox discovered while using `javascript_include_tag "application", async: false` as firefox defaults async to true when async attribute parameter is not included. This patch allows `javascript_include_tag "application", async: "false"` to print the attribute verbatim allowing workarounds for boolean values in tag attributes that default to true on exclusion.