Enable customing mime type and charset when returning reqeust string job.

browser/api/lib/protocol.coffee

@@ -1 +1,11 @@
 module.exports = process.atomBinding 'protocol'
+
+module.exports.RequestStringJob =
+class RequestStringJob
+  constructor: ({mimeType, charset, data}) ->
+    if typeof data isnt 'string' and not data instanceof Buffer
+      throw new TypeError('Data should be string or Buffer')
+
+    @mimeType = mimeType ? 'text/plain'
+    @charset = charset ? 'UTF-8'
+    @data = String data