Add support to Shakapacker

Shakapacker is the official, actively maintained successor to Webpacker.

Shakapacker v7 changed the spelling of `Webpacker` to `Shakapacker` in
the entire project, but still provided backward compatibility for
`Webpacker` spelling.

v8 dropped the deprecated spelling

This commit checks if Shakapacker is defined, otherwise it falls back
on Webpacker.

Ref: shakacode/shakapacker#429

Close jamesmartin#156

lib/inline_svg/webpack_asset_finder.rb

@@ -6,22 +6,31 @@ def self.find_asset(filename)
 
     def initialize(filename)
       @filename = filename
-      manifest_lookup = Webpacker.manifest.lookup(@filename)
+      manifest_lookup = wrapper.manifest.lookup(@filename)
       @asset_path =  manifest_lookup.present? ? URI(manifest_lookup).path : ""
     end
 
     def pathname
       return if @asset_path.blank?
 
-      if Webpacker.dev_server.running?
+      if wrapper.dev_server.running?
         dev_server_asset(@asset_path)
-      elsif Webpacker.config.public_path.present?
-        File.join(Webpacker.config.public_path, @asset_path)
+      elsif wrapper.config.public_path.present?
+        File.join(wrapper.config.public_path, @asset_path)
       end
     end
 
     private
 
+    def wrapper
+      @wrapper ||=
+        if defined?(::Shakapacker)
+          ::Shakapacker
+        else
+          ::Webpacker
+        end
+    end
+
     def dev_server_asset(file_path)
       asset = fetch_from_dev_server(file_path)
 
@@ -38,8 +47,8 @@ def dev_server_asset(file_path)
     end
 
     def fetch_from_dev_server(file_path)
-      http = Net::HTTP.new(Webpacker.dev_server.host, Webpacker.dev_server.port)
-      http.use_ssl = Webpacker.dev_server.https?
+      http = Net::HTTP.new(wrapper.dev_server.host, wrapper.dev_server.port)
+      http.use_ssl = wrapper.dev_server.https?
       http.verify_mode = OpenSSL::SSL::VERIFY_NONE
 
       http.request(Net::HTTP::Get.new(file_path)).body

spec/webpack_asset_finder_spec.rb

@@ -10,4 +10,14 @@
       expect(described_class.find_asset('some-file').pathname).to be_nil
     end
   end
+
+  context "when Shakapacker is defined" do
+    it "uses the new spelling" do
+      stub_const('Rails', double('Rails').as_null_object)
+      stub_const('Shakapacker', double('Shakapacker').as_null_object)
+      expect(::Shakapacker.manifest).to receive(:lookup).with('some-file').and_return(nil)
+
+      expect(described_class.find_asset('some-file').pathname).to be_nil
+    end
+  end
 end