Proxying to https correctly.

While working with https://secure.gravatar.com/xmlrpc I discovered that the proxy request was using http even when it was proxying to an https port / url.  This is because we didn't pass the https parameter through in the proxyRequest method.

This fix corrects as can be seen with the following code (assuming you're running cors-anywhere on 48080 on localhost).


```javascript
var str = '<?xml version="1.0"?>\
<methodCall>\
	<methodName>grav.test</methodName>\
	<params>\
		<param>\
			<value>\
			<struct>\
				<member>\
					<name>password</name><value><string>notMyRealPassword</string></value>\
				</member>\
			</struct>\
			</value>\
		</param>\
	</params>\
</methodCall>';
var xhr = new XMLHttpRequest();
var host = "http://localhost:48080"; // "https://cors-anywhere.herokuapp.com"
xhr.open('POST', host + '/https://secure.gravatar.com/xmlrpc?user=2a10f3086f0a55dcb3e05acdd9111a4f');
xhr.setRequestHeader('x-requested-with', 'my-code');
```

Before this change, I would receive a warning message 
```
400 Bad Request : The plain HTTP request was sent to HTTPS port
```

After the change, I get the expected XML back saying (since I passed the incorrect password) 

```xml
<?xml version="1.0"?>
	<methodResponse>
	   <fault>
		  <value>
			 <struct>
				<member>
				   <name>faultCode</name>
				   <value><int>-9</int></value>
				</member>
				<member>
				   <name>faultString</name>
				   <value><string>invalid or missing authentication information</string></value>
			   </member>
			</struct>
		 </value>
	  </fault>
</methodResponse>
```