Fix ENV.each_key implementation

Author: bjfish

CHANGELOG.md

@@ -55,6 +55,7 @@ Bug fixes:
 * Made `Kernel#caller_locations` accept a range argument, and return `nil` when appropriate.
 * Made `rb_respond_to` work with primitives (#1869, @chrisseaton).
 * Fixed issue with missing backtrace for `rescue $ERROR_INFO` (#1660).
+* 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