Correct way in new versions of nginx

Turn out my first answer to this question was correct at certain time, but it turned into another pitfall - to stay up to date please check Taxing rewrite pitfalls

I have been corrected by many SE users, so the credit goes to them, but more importantly, here is the correct code:

server {
       listen         80;
       server_name    my.domain.com;
       return         301 https://$server_name$request_uri;
}

server {
       listen         443 ssl;
       server_name    my.domain.com;
       # add Strict-Transport-Security to prevent man in the middle attacks
       add_header Strict-Transport-Security "max-age=31536000" always; 

       [....]
}

I would advise against changing the Zone settings as well. I resolved this issue using a small tweak to the XSL of the SharePoint RSS Web Part here:

Stop mixed content warnings in SharePoint's RSS Web Part

<xsl:template name="GetSafeHtml">
    <xsl:param name="Html"/>
    <xsl:choose>
        <xsl:when test="$rss_IsDesignMode = 'True'">
            <xsl:call-template name="strip-tags">
                <xsl:with-param name="text" select="$Html"/>
            </xsl:call-template>
        </xsl:when>
        <xsl:otherwise>
            <xsl:call-template name="strip-tags">
                <xsl:with-param name="text" select="rssaggwrt:MakeSafe($Html)"/>
            </xsl:call-template>
        </xsl:otherwise>
    </xsl:choose>
</xsl:template>

<xsl:template name="strip-tags">
    <xsl:param name="text"/>
    <xsl:choose>
        <xsl:when test="contains($text, '&lt;')">
            <xsl:value-of select="substring-before($text, '&lt;')"/>
            <xsl:call-template name="strip-tags">
                <xsl:with-param name="text" select="substring-after($text, '&gt;')"/>
            </xsl:call-template>
        </xsl:when>
        <xsl:otherwise>
            <xsl:value-of select="$text"/>
        </xsl:otherwise>
    </xsl:choose>
</xsl:template>

Basically, you paste over the GetSafeHtml template with a slightly modified version that calls a strip-tags template.

Good luck!