Move contents of core/env/shared/key.rb into its caller

This was shared between ENV.key and ENV.index, the latter one has been
removed in Ruby 3.0. This removed the need for warning suppression as
well.

core/env/key_spec.rb

@@ -1,11 +1,32 @@
 require_relative '../../spec_helper'
 require_relative 'shared/include'
-require_relative 'shared/key'
 
 describe "ENV.key?" do
   it_behaves_like :env_include, :key?
 end
 
 describe "ENV.key" do
-  it_behaves_like :env_key, :key
+  before :each do
+    @saved_foo = ENV["foo"]
+  end
+
+  after :each do
+    ENV["foo"] = @saved_foo
+  end
+
+  it "returns the index associated with the passed value" do
+    ENV["foo"] = "bar"
+    ENV.key("bar").should == "foo"
+  end
+
+  it "returns nil if the passed value is not found" do
+    ENV.delete("foo")
+    ENV.key("foo").should be_nil
+  end
+
+  it "raises TypeError if the argument is not a String and does not respond to #to_str" do
+    -> {
+      ENV.key(Object.new)
+    }.should raise_error(TypeError, "no implicit conversion of Object into String")
+  end
 end