Preserve existing source maps when adding line counts

CHANGELOG.md

@@ -1,5 +1,7 @@
 **Master**
 
+- Preserve existing source maps when adding line counts. [Oskar Eichler](https://github.com/OskarEichler)
+
 Get upgrade notes from Sprockets 3.x to 4.x at https://github.com/rails/sprockets/blob/master/UPGRADING.md
 
 # 4.4.1

README.md

@@ -42,6 +42,8 @@ If you want to work on Sprockets or better understand how it works read [How Spr
 
 ## Behavior Overview
 
+The default source-map preprocessor copies an existing map before adding line-count metadata, preserving caller-owned and frozen maps.
+
 You can interact with Sprockets primarily through directives and file extensions. This section covers how to use each of these things, and the defaults that ship with Sprockets.
 
 Since you are likely using Sprockets through another framework (such as the [Rails asset pipeline](http://guides.rubyonrails.org/asset_pipeline.html)), there will be configuration options you can toggle that will change behavior such as what directories or files get compiled. For that documentation you should see your framework's documentation.

lib/sprockets/preprocessors/default_source_map.rb

@@ -26,7 +26,7 @@ def call(input)
             "names"     => []
           }
         else
-          result[:map] = map
+          result[:map] = map.dup
         end
 
         result[:map]["x_sprockets_linecount"] = lines