Unconditionally return the unhandledPromptBehavior capability

When creating a session, ensure that we always return the used value
of this capability, whether or not it was passed in.

Author: jgraham

index.html

@@ -2304,8 +2304,7 @@ <h3>Global State</h3>
   <li><p>Let <var>serialized user prompt handler</var>
    be <a>serialize the user prompt handler</a>.
 
-  <li><p>If <var>serialized user prompt handler</var> is not null, set
-   a property on <var>capabilities</var> with the name
+  <li><p>Set a property on <var>capabilities</var> with the name
    "<code>unhandledPromptBehavior</code>", and the value <var>serialized
    user prompt handler</var>.
 
@@ -10583,7 +10582,8 @@ <h3>User Prompt Handler</h3>
 <p>To <dfn>serialize the user prompt handler</dfn>:
 
 <ol class=algorithm>
- <li><p>If the <a>user prompt handler</a> is null, return null.
+ <li><p>If the <a>user prompt handler</a> is null, return
+ "<code>dismiss and notify</code>".
 
  <li><p>If the <a>user prompt handler</a> has [=map/size=] 1,
  and <a>user prompt handler</a> [=list/contains=]