[changelog skip] Quote buildpack's export values

This is a followup from #965.

In #888 the values in the buildpack's `export` file dropped their quoting for paths causing a few failures that were not caught by our tests.

This PR fixes the underlying problem and adds a test that would have caught the regression. The test uses a buildpack that intentionally adds a quoted newline to it's export file https://github.com/sharpstone/export_path_with_newlines_buildpack/blob/master/bin/compile this gets picked up by the Ruby buildpack and put back into Ruby's `export` file. Since ruby after #888 did not quote these values it would result in this failure:

```
  1) Buildpack internals handles PATH with a newline in it correctly
     Failure/Error:
       Hatchet::Runner.new("default_ruby", buildpacks: buildpacks).deploy do |app|
         expect(app.output).to_not match("No such file or directory")
       end
     Hatchet::App::FailedDeploy:
       Could not deploy 'hatchet-t-3e6bacb18a' (default_ruby) using 'Hatchet::GitApp' at path: './repos/rack/default_ruby'
        if this was expected add `allow_failure: true` to your deploy hash.
       output:
       Buildpack: nil
       Repo: https://git.heroku.com/hatchet-t-3e6bacb18a.git
       remote: Compressing source files... done.        
       remote: Building source:        
       remote: 
       remote: -----> I add newlines to the export file app detected        
       remote: -----> Ruby app detected        
       remote: -----> Installing bundler 1.17.3        
       remote: -----> Removing BUNDLED WITH version in the Gemfile.lock        
       remote: -----> Compiling Ruby/Rack        
       remote: -----> Using Ruby version: ruby-2.6.5        
       remote: -----> Installing dependencies using bundler 1.17.3        
       remote:        Running: bundle install --without development:test --path vendor/bundle --binstubs vendor/bundle/bin -j4 --deployment        
       remote:        Fetching gem metadata from https://rubygems.org/...............        
       remote:        Using bundler 1.17.3        
       remote:        Fetching rack 1.5.0        
       remote:        Installing rack 1.5.0        
       remote:        Bundle complete! 1 Gemfile dependency, 2 gems now installed.        
       remote:        Gems in the groups development and test were not installed.        
       remote:        Bundled gems are installed into `./vendor/bundle`        
       remote:        Bundle completed (3.14s)        
       remote:        Cleaning up the bundler cache.        
       remote: -----> Writing config/database.yml to read from DATABASE_URL        
       remote: -----> Detecting rake tasks        
       remote: 
       remote: ###### WARNING:        
       remote: 
       remote:        You have not declared a Ruby version in your Gemfile.        
       remote:        To set your Ruby version add this line to your Gemfile:        
       remote:        ruby '2.6.5'        
       remote:        # See https://devcenter.heroku.com/articles/ruby-versions for more information.        
       remote: 
       remote: ###### WARNING:        
       remote: 
       remote:        No Procfile detected, using the default web server.        
       remote:        We recommend explicitly declaring how to boot your server process via a Procfile.        
       remote:        https://devcenter.heroku.com/articles/ruby-default-web-server        
       remote: 
       remote: 
       remote: -----> Null Buildpack app detected        
       remote: /app/tmp/buildpacks/6ccd2b9f41db985dbeffeeafa11877f50b22197988b0d84a055bdf7fe48efeff0c1fbccefc0b0ef5dd833ede16523bd9cec929eeebc3e0e3bfe67c0f12460db2/export: line 2: /dog:/tmp/build_fdb4f5eaca8a5eb93660ff3af68703df/hello/there:/usr/local/bin:/usr/bin:/bin:/tmp/codon/vendor/bin:/tmp/build_fdb4f5eaca8a5eb93660ff3af68703df/bin:/usr/local/bin:/usr/bin:/bin:/tmp/build_fdb4f5eaca8a5eb93660ff3af68703df/./vendor/bundle/bin:/tmp/build_fdb4f5eaca8a5eb93660ff3af68703df/./vendor/bundle/ruby/2.6.0/bin:/tmp/build_fdb4f5eaca8a5eb93660ff3af68703df/vendor/ruby-2.6.5/bin:/tmp/tmp.HN41keQoC9/bin/:cinco/: No such file or directory        
       remote: /usr/bin/env: ‘bash’: No such file or directory        
       remote:  !     Push rejected, failed to compile Null Buildpack app.        
       remote: 
       remote:  !     Push failed        
       remote: Verifying deploy...        
       remote: 
       remote: !	Push rejected to hatchet-t-3e6bacb18a.        
       remote: 

```

lib/language_pack/base.rb

@@ -253,9 +253,9 @@ def export(key, val, layer: nil, option: nil)
     else
       string =
         if option == :default
-          "export #{key}=${#{key}:-#{val}}"
+          %{export #{key}="${#{key}:-#{val}}"}
         elsif option == :path
-          "export #{key}=#{val}:$#{key}"
+          %{export #{key}="#{val}:$#{key}"}
         else
           %{export #{key}="#{val.gsub('"','\"')}"}
         end

spec/hatchet/buildpack_spec.rb

@@ -0,0 +1,15 @@
+require_relative '../spec_helper'
+
+describe "Buildpack internals" do
+  it "handles PATH with a newline in it correctly" do
+    buildpacks = [
+      "https://github.com/sharpstone/export_path_with_newlines_buildpack",
+      Hatchet::App.default_buildpack,
+      "https://github.com/heroku/null-buildpack"
+    ]
+    Hatchet::Runner.new("default_ruby", buildpacks: buildpacks).deploy do |app|
+      expect(app.output).to_not match("No such file or directory")
+    end
+  end
+end
+