[policies] Groovy policy error on HTTP headers

[ytvnr]

💥 Describe the bug

Since the modification of HTTP Headers in 3.15.x, accessing a header through a groovy script via request.headers.'My-Header' will return a string instead of an array of values, meaning it becomes impossible to do array operation (join for example).

🌄 To Reproduce

Steps to reproduce the behaviour:
Add a groovy script to your policy and use:

if (request.headers.containsKey('test')) {
     userAgent = request.headers.'test'.join('|');
request.headers.'test2' = userAgent;

   }

It should fail with

22:05:55.485 \[vert.x-eventloop-thread-1\] \[\] ERROR i.g.policy.groovy.GroovyPolicy - Unable to run Groovy script  
java.lang.SecurityException: Failed to resolve method \[ class java.lang.String join java.lang.String \]  
        at io.gravitee.policy.groovy.sandbox.SecuredInterceptor.onMethodCall(SecuredInterceptor.java:49)

🌈 Expected behaviour

We should be able to join all the values in a string if we want

[mouligno]

LGTM