Change headers (exercism#37)

* Remove content-length header

* Change content-type to text/plain

Author: ErikSchierboom

Diff for: lib/snippet_extractor/process_request.rb

@@ -10,10 +10,7 @@ def call
       {
         statusCode: 200,
         statusDescription: "200 OK",
-        headers: {
-          'Content-Length': snippet.bytesize,
-          'Content-Type': 'application/plain; charset=utf-8'
-        },
+        headers: { 'Content-Type': 'text/plain' },
         isBase64Encoded: false,
         body: snippet
       }

Diff for: test/process_request_test.rb

@@ -23,10 +23,7 @@ def test_e2e
       expected = {
         statusCode: 200,
         statusDescription: "200 OK",
-        headers: {
-          'Content-Length': 9,
-          'Content-Type': 'application/plain; charset=utf-8'
-        },
+        headers: { 'Content-Type': 'text/plain' },
         isBase64Encoded: false,
         body: snippet.rstrip
       }