Clarify that account must have a username to be able to set HTTP password

Update the REST API documentation to clarify that an account must have
its username set to be able to set the HTTP password.

Also add a test for this scenario.

Change-Id: Id46cd8abcdea12f002c5aa1ff18ce323e7338fb3

Documentation/rest-api-accounts.txt

diff --git a/Documentation/rest-api-accounts.txt b/Documentation/rest-api-accounts.txt
index dc45ff5..590b534 100644
--- a/Documentation/rest-api-accounts.txt
+++ b/Documentation/rest-api-accounts.txt
@@ -473,6 +473,8 @@
 in the request body inside a link:#http-password-input[
 HttpPasswordInput] entity.
 
+The account must have a username.
+
 .Request
 ----
   PUT /accounts/self/password.http HTTP/1.0