Fix IO#write to not modify the argument when the encoding does not match

Author: eregon

CHANGELOG.md

@@ -5,6 +5,7 @@ Bug fixes:
 * Set `RbConfig::CONFIG['ruby_version']` to the same value as the TruffleRuby version. This fixes reusing C extensions between different versions of TruffleRuby with Bundler (#1715).
 * Fixed `Symbol#match` returning `MatchData` (#1706).
 * Allow `Time#strftime` to be called with binary format strings.
+* Do not modify the argument passed to `IO#write` when the encoding does not match (#1714).
 
 Compatibility:
 

src/main/ruby/truffleruby/core/io.rb

@@ -2549,7 +2549,7 @@ def write(data)
        external_encoding != data.encoding &&
        external_encoding != Encoding::ASCII_8BIT
       unless data.ascii_only? && external_encoding.ascii_compatible?
-        data.encode!(external_encoding)
+        data = data.encode(external_encoding)
       end
     end