Yhyjyk

This is the full error:

System.XmlException: start tag unexpected character = (position: END_TAG seen ...<source> Bicycling</source><media:contenturl=... @1:688)

So from my understanding, this is not valid XML. I was having an issue with the session body and had to make the following change:

<rss xmlns:media="http:..." and had to add <rsstxlmns:media="https:..."

Now, I am not sure how to go about this with the = sign.

@isTest
 global class getRSSFeedMockCallout implements HttpCalloutMock 


 global HttpResponse respond(HttpRequest request)
 
 String sessionBody = '<rsstxmlns:media="http://search.yahoo.com/mrss/"version="2.0">' +
 '<channel>'+
 '<item>'+
 '<title>Draymond Green Is a SoulCycle Devotee - Bicycling</title>'+
 '<link>https://www.bicycling.com/news/a27024393/draymond-green-soulcycle/</link>'+
 '<pubDate>Wed, 03 Apr 2019 12:00:00 GMT</pubDate>'+
 '<description><a href="https://www.bicycling.com/news/a27024393/draymond-green-soulcycle/"target="_blank">Draymond Green Is a SoulCycle Devotee</a>&nbsp;&nbsp;<font color="#6f6f6f">Bicycling</font><p>If you take SoulCycle classes in the Bay Area, and you notice one extremely tall guy in the room, you may be looking at Golden State Warriors star Draymond ...</p></description>'+
 '<source>Bicycling</source>'+
 '<media:contenturl="https://lh5.googleusercontent.com/proxy/QvLCgDJMIqLENGFBUt1CCoUPO1XB6gJkGFPV5FuM0QKfyaG8WdhuHGtfcVaTzNzPAYyHo-arnqUBkWkZ8s7-p7WBUbKlfJ4xr0hl8hltQWsBCG0yDDh13Ul5qKrZgLvjQitRecmu=-w150-h150-c"medium="image"width="150"height="150"/>'+
 '</item>'+
 '</channel>'+
 '</rss>';


 HttpResponse res = new HttpResponse();
 res.setHeader('Content-Type', 'text/xml');
 res.setBody( sessionBody );
 res.setStatusCode(200);
 return res;
 

There are few things wrong with the xml.

1) In Apex, the string starts and end with single quotes ' , thus you don't need to escape "

2) You cannot use & in XML, it makes XML invalid, you have to escape it.   will become  

3) There is no tag as media:contenturl in rss specification ,the tag is media:content and having url as an attribute. Seems like you missed a space in that.

Thus fixing all the above problem and formating the code, the code would be something like

String input = '<rss xmlns:media="http://search.yahoo.com/mrss/" version="2.0">' +
 '<channel>' +
 '<item>' +
 '<title>Draymond Green Is a SoulCycle Devotee - Bicycling</title>' +
 '<link>https://www.bicycling.com/news/a27024393/draymond-green-soulcycle/</link>' +
 '<pubDate>Wed, 03 Apr 2019 12:00:00 GMT</pubDate>' +
 '<description>' +
 '<a href="https://www.bicycling.com/news/a27024393/draymond-green-soulcycle/" target="_blank">Draymond Green Is a SoulCycle Devotee &amp;nbsp;&amp;nbsp; </a>' +
 '<font color="#6f6f6f">Bicycling</font>' +
 '<p>If you take SoulCycle classes in the Bay Area, and you notice one extremely tall guy in the room, you may be looking at Golden State Warriors star Draymond ...</p>' +
 '</description>' +
 '<source>Bicycling</source>' +
 '<media:content url ="https://lh5.googleusercontent.com/proxy/QvLCgDJMIqLENGFBUt1CCoUPO1XB6gJkGFPV5FuM0QKfyaG8WdhuHGtfcVaTzNzPAYyHo-arnqUBkWkZ8s7-p7WBUbKlfJ4xr0hl8hltQWsBCG0yDDh13Ul5qKrZgLvjQitRecmu=-w150-h150-c" medium="image" width="150" height="150"/>' +
 '</item>' +
 '</channel>' +
 '</rss>';


System.debug(input);