fix platform-specific compression test failure

The problem is that gzip headers include a platform identifier.
Previously, zlib was incorrectly treating macOS as just a "generic
UNIX", but now it is actually using the correct header. The updated zlib
made its way in to recent versions of node and started uncovering a new
source of platform-specific assumptions that people didn't know they
were making.

To fix this, instead of hard-coding the expected gzip compression
result, we can just generate them on the testing platform.

This is an instance of nodejs/node#12244.

test/invoke.policy.http.test.js

@@ -10,6 +10,7 @@ var backend = require('./support/invoke-server');
 var apimServer = require('./support/mock-apim-server/apim-server');
 var dsCleanup = require('./support/utils').dsCleanup;
 var resetLimiterCache = require('../lib/rate-limit/util').resetLimiterCache;
+var zlib = require('zlib');
 
 describe('invokePolicy', function() {
   var request;
@@ -287,14 +288,20 @@ describe('invokePolicy', function() {
   it('compress-data', function(done) {
     this.timeout(10000);
 
+    // gzip compression header includes a platform bit which means different
+    // platforms actually produce _slightly_ different gzip compression results
+    // depending on the platform.
+    // see https://github.com/nodejs/node/issues/12244
+    const gzHelloWorld = zlib.gzipSync('Hello World').toString('base64');
+
     // when data is compressed, use the chunked encoding
     request
       .post('/invoke/testCompression')
       .set('X-RAW-DATA', 'Hello World')
       .expect(/z-content-encoding: gzip/)
       .expect(/z-content-length: undefined/)
       .expect(/z-transfer-encoding: chunked/)
-      .expect(/raw: H4sIAAAAAAAAA\/NIzcnJVwjPL8pJAQBWsRdKCwAAAA==/)
+      .expect(new RegExp(`raw: ${gzHelloWorld}`))
       .expect(200, /body: Hello World/, done);
   });