Match static file extensions case-insensitively

[xop01]

Summary

• file_extension keeps the original case, and find_content_type switched on lowercase str2tag literals. index.HTML and a.JPG therefore fell through to application/octet-stream.
• The extension is lowercased before the user-map lookup and the built-in switch. set_file_extension_and_mimetype_mapping stores the key in lowercase too, so a mapping registered as HTML matches .html and .HTML.

Test plan

• find_content_type("dir/index.HTML") is text/html, and a.JPG is image/jpeg
• A user map keyed by abcde matches f.ABCDE
• FindContentTypeTest.ExtensionCaseIsIgnored
• ServerTest.UserDefinedMIMETypeMapping still passes

[yhirose]

Thanks for the PR. Matching extensions case-insensitively would change the Content-Type served for existing files, which affects current users. I'd prefer to keep the current behavior unless there is a concrete need. Closing.