Force encoding of the context for the response of an SSI include

We use SSI to insert frequently changing text parts on our side (e.g. price changes).
At the moment we have a .txt file on CloudFront with the content "€254".

When I try to include that file I get the error:
incompatible character encodings: UTF-8 and ASCII-8BIT

Traceback:
rack_ssi (0.0.2) lib/ssi_processor.rb:28:in `gsub'
rack_ssi (0.0.2) lib/ssi_processor.rb:28:in `process_include'
rack_ssi (0.0.2) lib/ssi_processor.rb:13:in `block in process'
...

The ngnix SSI implementation works fine.

My solution is to force the encoding of the context for the response of an SSI include.

Cheers,
Thomas