Fix duplicate view template loading

[niklas-hasselmeyer]

Motivation / Background

Currently, when rendering an ActionView::Template, the template source is loaded from the disk twice.

During one rendering process, Template#source is called at least twice (once in encode! and once in strict_locals!), each of these calls runs @source.to_s. As @source is usually a Template::Sources::File, #to_s hits the disk to load the template.

It is unnecessary to load the file from disk more than once just to check whether the # locals: comment is present.

Detail

This Pull Request changes the Template#source so that it memoizes the loaded source.

Additional information

This pull request introduces a small behavior change: The # locals: (...) comment is now removed from the source before passing the source to the handler. It looks like this was the intended behavior all along. Previously the line

self.source.sub!(STRICT_LOCALS_REGEX, "")

had no effect, because the source was reloaded from disk afterwards and the magic comment was present again.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.