Do not set variable GZip#mtime but do always set File#mtime - #821

This revisits the discussion in #197 to handle gzipped assets appropriately in the context of web assets.

• Sets the GZip mtime header value to a fixed value. By spec this should be 0, but Ruby < 2.7 had a ZLib bug that caused a 0 value to be equivalent to unset which results in it being set to the current time. So 1 is used instead. The same compromise was just merged into Rack::Deflater.
• Sets the correct File mtime for both GZip and Zopfli compressed files. It looks like that was overlooked when Zopfli support was added.

Why

The GZip mtime is a header value of the compressed file; changing the header file changes the contents (content integrity hash) of the file. That's annoying when writing to Docker or Git (#707) that just look at the file contents and can affect caching/cacheability (not strictly Conditional Requests). It's not necessary to set this GZip header to an accurate value. I documented this more over in the Rack::Deflater PR, but briefly:

• Apache mod_deflate sets gzip mtime to 0
• nginx ngx_http_gzip_module uses deflateInit2 with a setting that sets mtime to 0
• Zopfli sets the header to zero (code).

We do want to set the File mtime because that is used for setting the last-modified http header, and this ensures that the uncompressed source and the compressed file share the same last-modified value. This change aligns the lifecycle of the uncompressed and compressed files so that both last-modified and file contents change together.