Fix Encoding::UndefinedConversionError in activation.rb

[mckeed]

Motivation

Another fix for #3200. The last one only checked for malformed UTF-8, not correct UTF-8 with ASCII encoding, which is the issue.

Implementation

I changed it so it converts everything to UTF-8 to avoid the Encoding::UndefinedConversionError error.

Automated Tests

I updated the test so it fails on the previous fix.

Manual Tests

On macOS, add a unicode character to your system shell prompt and launch vscode using the .app, not from the command line.

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[webmatze]

@mckeed I tested the change on my system, and the problem is now solved, at least for me!

[andyw8]

(nit) Let's split this to make a it a little more readable:

Suggested change

	env = ENV.filter_map { |k, v| v = v.dup.force_encoding(Encoding::UTF_8); "#{k}RUBY_LSP_VS#{v}" if v.valid_encoding? }
	env = ENV.filter_map do |k, v|
	v = v.dup.force_encoding(Encoding::UTF_8)
	"#{k}RUBY_LSP_VS#{v}" if v.valid_encoding?
	end