Fix postsign (#4347)

* Fix whitespace.

* Fix post sign.

* Fix message.

* Fix tests.

* Rest of the problems.

* All hail the linter and its omniscience.

* ...and its divine omniscience.

* Grumbles and wording.

js/src/api/rpc/parity/parity.js

@@ -297,6 +297,11 @@ export default class Parity {
       .execute('parity_postTransaction', inOptions(options));
   }
 
+  postSign (from, message) {
+    return this._transport
+      .execute('parity_postSign', from, message);
+  }
+
   registryAddress () {
     return this._transport
       .execute('parity_registryAddress')