Fixed #1 (Source file is transformed).

lib/index.js

@@ -167,7 +167,10 @@ function plugin (options) {
       return new Promise((resolve, reject) => {
         let dir = path.dirname(file),
             destination = path.basename(file, path.extname(file)) + '.' + pluginOptions.device,
-            data = files[file],
+            data = {
+              contents: Buffer.from(files[file].contents),
+              mode: files[file].mode
+            }
             input = data.contents
 
         if (dir !== '.')

test/fixtures/source/src/foo.ms

@@ -0,0 +1 @@
+foo

test/index.js

@@ -130,4 +130,25 @@ describe('metalsmith-groff', function () {
                          'Preprocessors are missing: ' + Object.values(preprocessors))
     })
   })
+
+  it('should keep the source file if requested to do so', function (done) {
+    const metalsmith = Metalsmith('test/fixtures/source'),
+          logger = debug('metalsmith-groff:test:source')
+
+    metalsmith
+      .use(groff({
+        exe: 'test/groff_mock',
+        source: true
+      }))
+      .use((files, metalsmith) => {
+        assert.equal(Object.keys(files).length, 2, 'Expected two files on output!')
+        assert.equal(files['foo.ms'].contents.toString('utf8'), 'foo',
+                     'Source file was modified!')
+      })
+      .process((err) => {
+        if (err)
+          throw err
+        done()
+      })
+  })
 })