Fix falsely passing specs looking for "shortval"

They were finding it in the exception backtrace that was being dumped to the HTML.

better_errors.gemspec

@@ -22,6 +22,7 @@ Gem::Specification.new do |s|
 
   s.add_development_dependency "rake", "~> 10.0"
   s.add_development_dependency "rspec", "~> 3.5"
+  s.add_development_dependency "rspec-html-matchers"
   s.add_development_dependency "rspec-its"
   s.add_development_dependency "yard"
   # kramdown 2.1 requires Ruby 2.3+

spec/better_errors/error_page_spec.rb

@@ -2,6 +2,11 @@
 
 module BetterErrors
   describe ErrorPage do
+    # It's necessary to use HTML matchers here that are specific as possible.
+    # This is because if there's an exception within this file, the lines of code will be reflected in the
+    # generated HTML, so any strings being matched against the HTML content will be there if they're within 5
+    # lines of code of the exception that was raised.
+
     let!(:exception) { raise ZeroDivisionError, "you divided by zero you silly goose!" rescue $! }
 
     let(:error_page) { ErrorPage.new exception, { "PATH_INFO" => "/some/path" } }
@@ -104,7 +109,7 @@ def inspect
 
                 it "shows the variable content" do
                   html = error_page.do_variables("index" => 0)[:html]
-                  expect(html).to include("shortval")
+                  expect(html).to have_tag('div.variables', text: /shortval/)
                 end
               end
               context 'and does not implement #inspect' do
@@ -185,7 +190,7 @@ def inspect
 
                 it "shows the variable content" do
                   html = error_page.do_variables("index" => 0)[:html]
-                  expect(html).to include("shortval")
+                  expect(html).to have_tag('div.variables', text: /shortval/)
                 end
               end
               context 'and does not implement #inspect' do
@@ -198,8 +203,8 @@ def inspect
 
                 it "includes an indication that the variable was too large" do
                   html = error_page.do_variables("index" => 0)[:html]
-                  expect(html).to_not include(content)
-                  expect(html).to include("Object too large")
+                  expect(html).not_to have_tag('div.variables', text: %r{#{content}})
+                  expect(html).to have_tag('div.variables', text: /Object too large/)
                 end
               end
             end

spec/spec_helper.rb

@@ -19,3 +19,9 @@
 
 require 'bundler/setup'
 Bundler.require(:default)
+
+require 'rspec-html-matchers'
+
+RSpec.configure do |config|
+  config.include RSpecHtmlMatchers
+end