Detail how javascript: return values become response bodies

Fixes #301, by aligning with the 3/4 browser majority and checking the
type of the completion value, turning non-strings and thrown errors into
204s. (Thrown errors are still reported, however.)

While working on this algorithm, we fix #945 by copying the HTTPS state
to response.

This also does some minor cleanup to clarify that "run a classic script"
returns undefined when scripting is disabled.

#1129 was opened to track a remaining open issue discovered, which is
exactly how the JS string completion value becomes a response body. For
now the spec includes a warning saying that this is underspecified.

source

@@ -82536,9 +82536,8 @@ State: <OUTPUT NAME=I>1</OUTPUT> <INPUT VALUE="Increment" TYPE=BUTTON O
 
        <li><p>If the <span>origin</span> of the <span>source browsing context</span> is not the
        <span>same origin</span> as the <span>origin</span> of the <span>active document</span> of
-       the <span>browsing context</span> being navigated, then act as if the result of evaluating
-       the script was the void value, and jump to the step labeled <i>process results</i>
-       below.</p></li>
+       the <span>browsing context</span> being navigated, then let <var>result</var> be undefined,
+       and jump to the step labeled <i>process result</i> below.</p></li>
 
        <li><p>Let <var>urlRecord</var> be the result of running the <span>URL parser</span> on the
        <span>URL</span> being navigated.</p></li>
@@ -82584,22 +82583,28 @@ State: &lt;OUTPUT NAME=I>1&lt;/OUTPUT> &lt;INPUT VALUE="Increment" TYPE=BUTTON O
 
        <li><p>Let <var>result</var> be the result of <span data-x="run a classic script">running the
        classic script</span> <var>script</var>. If evaluation was unsuccessful, let
-       <var>result</var> be void instead. (The result will be void also if <span
+       <var>result</var> be undefined instead. (The result will also be undefined if <span
        data-x="concept-bc-noscript">scripting is disabled</span>.)</p></li>
 
        <li>
 
-        <p><i>Process results</i>: If <var>result</var> is void (there is no return value), then
-        the result of obtaining the resource for the URL is a <span
+        <p><i>Process result</i>: If <span data-x="js-Type">Type</span>(<var>result</var>) is not
+        String, then the result of obtaining the resource for the URL is a <span
         data-x="concept-response">response</span> whose <span
         data-x="concept-response-status">status</span> is <code data-x="">204</code>.</p>
 
         <p>Otherwise, the result of obtaining the resource for the URL is a <span
         data-x="concept-response">response</span> whose <span
         data-x="concept-response-header-list">header list</span> consists of <code
-        data-x="">Content-Type</code>/<code>text/html</code> and whose <span
-        data-x="concept-response-body">body</span> is the return value converted to a string
-        value.</p>
+        data-x="">Content-Type</code>/<code>text/html</code>, whose <span
+        data-x="concept-response-body">body</span> is <var>result</var>, and whose <span
+        data-x="concept-response-https-state">HTTPS state</span> is <var>settings</var>'s
+        <span>HTTPS state</span>.</p>
+
+        <p class="warning">The exact conversion between the JavaScript string <var>result</var> and
+        the bytes that comprise a <span data-x="concept-response-body">response body</span> is
+        not yet specified, pending further investigation into user agent behavior. See <a
+        href="https://github.com/whatwg/html/issues/1129">issue #1129</a>.</p>
 
         <p>When it comes time to <span>set the document's address</span> in the <span
         data-x="navigate">navigation algorithm</span>, use <var>address</var> as the
@@ -87358,8 +87363,8 @@ interface <dfn>NavigatorOnLine</dfn> {
 
    <li><p>Let <var>settings</var> be the <span>settings object</span> of <var>s</var>.</p></li>
 
-   <li><p><span>Check if we can run script</span> with <var>settings</var>. If this returns "do
-   not run" then abort these steps.</p></li>
+   <li><p><span>Check if we can run script</span> with <var>settings</var>. If this returns "do not
+   run", then return undefined and abort these steps.</p></li>
 
    <li><p>Let <var>realm</var> be <var>settings</var>'s <span
    data-x="environment settings object's Realm">Realm</span>.</p></li>