Make sure that ENV.[]= fails when an invalid variable name is given.

Fix this if there is an environment where a variable name containing
an '=' is allowed.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@26285 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_env.rb

@@ -122,6 +122,7 @@ class TestEnv < Test::Unit::TestCase
     assert_equal(nil, ENV["test"])
     assert_raise(ArgumentError) { ENV["foo\0bar"] = "test" }
     assert_raise(ArgumentError) { ENV["test"] = "foo\0bar" }
+    assert_raise(Errno::EINVAL) { ENV["foo=bar"] = "test" }
     ENV[PATH_ENV] = "/tmp/".taint
     assert_equal("/tmp/", ENV[PATH_ENV])
   end