Add or refresh specs that we are already passing

[#217]

spec/core/string/element_reference_spec.rb

@@ -0,0 +1,35 @@
+require_relative '../../spec_helper'
+require_relative 'fixtures/classes'
+require_relative 'shared/slice'
+
+describe "String#[]" do
+  it_behaves_like :string_slice, :[]
+end
+
+describe "String#[] with index, length" do
+  it_behaves_like :string_slice_index_length, :[]
+end
+
+describe "String#[] with Range" do
+  it_behaves_like :string_slice_range, :[]
+end
+
+describe "String#[] with Regexp" do
+  it_behaves_like :string_slice_regexp, :[]
+end
+
+describe "String#[] with Regexp, index" do
+  it_behaves_like :string_slice_regexp_index, :[]
+end
+
+describe "String#[] with Regexp, group" do
+  it_behaves_like :string_slice_regexp_group, :[]
+end
+
+describe "String#[] with String" do
+  it_behaves_like :string_slice_string, :[]
+end
+
+describe "String#[] with Symbol" do
+  it_behaves_like :string_slice_symbol, :[]
+end

spec/core/string/force_encoding_spec.rb

@@ -1,3 +1,4 @@
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 
 describe "String#force_encoding" do