Global response defaults

Hi Tom, as discussed, configuring global response values for matched request URLs.

Currently only response headers are affected.  Existing headers for a specific mapping are left unchanged, and new response headers are added.

Example global mapping:

{  
    "request": {                                    
        "method": "GET",                        
        "urlPattern": "/services/guest/v1/(.*)/(.*)",
        "headers": {
            "Accept": {
                "equalTo": "application/json"
            }
        }
    },                                       
    "globalDefaults": {                               
        "headers": {
            "Content-Type": "application/json",
            "Cache-Control": "max-age=10" 
        }
    }                                               
}