Fix updating of indexes containing escaped characters (#156)

There are a number of different mechanisms with
[String#sub](https://ruby-doc.org/core-2.4.5/String.html#method-i-sub)
that pertain to the matching of backreferences and capture groups when
using either the String or Hash type args for the replacement arg.

>If replacement is a String that looks like a pattern's capture group
but is actually not a pattern capture group e.g. "\'", then it will have
to be preceded by two backslashes like so "\\'".

When an index contains unescaped characters, those are being incorrectly
interpreted as capture groups. Using the block version of #sub avoids
this.

For example, here's the behaviour we see with an index containing
special characters that need escaping.

This becomes a problem given that the issue is in the `Updater` itself;
we get stuck in a loop where annotation changes are always being
detected, so a CI step with `--frozen` will always fail.

lib/annotate_rb/model_annotator/annotated_file/updater.rb

@@ -20,7 +20,7 @@ def update
 
           new_annotation = wrapped_content(@new_annotations)
 
-          _content = @file_content.sub(@parsed_file.annotations, new_annotation)
+          _content = @file_content.sub(@parsed_file.annotations) { new_annotation }
         end
 
         private

spec/lib/annotate_rb/model_annotator/annotated_file/updater_spec.rb

@@ -79,6 +79,61 @@ class User < ApplicationRecord
       end
     end
 
+    context "with an index change containing escaped characters" do
+      let(:file_content) do
+        <<~FILE
+          # == Schema Information
+          #
+          # Table name: users
+          #
+          #  id               :integer          not null, primary key
+          #  foreign_thing_id :integer          not null
+          #
+          class User < ApplicationRecord
+          end
+        FILE
+      end
+      let(:new_annotations) do
+        <<~ANNOTATIONS
+          # == Schema Information
+          #
+          # Table name: users
+          #
+          #  id               :integer          not null, primary key
+          #  foreign_thing_id :integer          not null
+          #
+          # Indexes
+          #
+          #  index_rails_02e851e3b8  (another_column) WHERE value IS LIKE '\\\\%'
+          #
+        ANNOTATIONS
+      end
+
+      let(:options) { AnnotateRb::Options.new({position_in_class: "before", show_foreign_keys: true}) }
+
+      let(:expected_content) do
+        <<~CONTENT
+          # == Schema Information
+          #
+          # Table name: users
+          #
+          #  id               :integer          not null, primary key
+          #  foreign_thing_id :integer          not null
+          #
+          # Indexes
+          #
+          #  index_rails_02e851e3b8  (another_column) WHERE value IS LIKE '\\\\%'
+          #
+          class User < ApplicationRecord
+          end
+        CONTENT
+      end
+
+      it "returns the updated annotated file" do
+        is_expected.to eq(expected_content)
+      end
+    end
+
     context 'when position is "after" for the existing annotation but position is "before" for the new annotation' do
       let(:file_content) do
         <<~FILE