[GR-20429] Fix ENV.each_key implementation.

PullRequest: truffleruby/1238

Author: bjfish

CHANGELOG.md

@@ -65,6 +65,7 @@ Bug fixes:
 * Removed "shadowing outer local variable" warning.
 * Fixed parameter conversion to `String` in ENV methods.
 * Fixed deprecation warning when `ENV.index` is called.
+* Fixed issue with `ENV.each_key`.
 
 Compatibility:
 

spec/tags/core/env/each_key_tags.txt

@@ -113,8 +113,12 @@ def each
   end
   alias_method :each_pair, :each
 
-  def each_key(&block)
-    @variables.each(&block)
+  def each_key
+    return to_enum(:each_key) { size } unless block_given?
+    @variables.each do |name|
+      yield set_encoding(name)
+    end
+    self
   end
 
   def each_value