StringIO#set_encoding should coerce the argument to an Encoding

* Fixes #2954

Author: eregon

CHANGELOG.md

@@ -117,6 +117,7 @@ Compatibility:
 * Add `Refinement#import_methods` method and add deprecation warning for `Refinement#include` and `Refinement#prepend` (#2733, @horakivo).
 * Upgrading `UNICODE` version to 13.0.0 and `EMOJI` version to 13.1 (#2733, @horakivo).
 * Add `rb_io_maybe_wait_readable`, `rb_io_maybe_wait_writable` and `rb_io_maybe_wait` functions (#2733, @andrykonchin).
+* `StringIO#set_encoding` should coerce the argument to an Encoding (#2954, @eregon).
 
 Performance:
 

lib/truffle/stringio.rb

@@ -191,7 +191,7 @@ def initialize_copy(from)
   end
 
   def set_encoding(external, internal = nil, options = nil)
-    encoding = external || Encoding.default_external
+    encoding = Truffle::Type.coerce_to_encoding(external || Encoding.default_external)
     d = @__data__
     TruffleRuby.synchronized(d) do
       d.encoding = encoding

spec/ruby/library/stringio/set_encoding_spec.rb

@@ -17,4 +17,12 @@
     io.set_encoding Encoding::UTF_8
     io.string.encoding.should == Encoding::US_ASCII
   end
+
+  it "accepts a String" do
+    str = "".encode(Encoding::US_ASCII)
+    io = StringIO.new(str)
+    io.set_encoding("ASCII-8BIT")
+    io.external_encoding.should == Encoding::BINARY
+    str.encoding.should == Encoding::BINARY
+  end
 end