Added session length option for session tokens to server configuration

spec/index.spec.js

@@ -280,4 +280,37 @@ describe('server', () => {
     }) ).toThrow("publicServerURL should be a valid HTTPS URL starting with https://");
     done();
   });
+
+  it('fails if the session length is not a number', (done) => {
+    expect(() => setServerConfiguration({
+      serverURL: 'http://localhost:8378/1',
+      appId: 'test',
+      appName: 'unused',
+      javascriptKey: 'test',
+      masterKey: 'test',
+      sessionLength: 'test'
+    })).toThrow('Session length must be a valid number.');
+    done();
+  });
+
+  it('fails if the session length is less than or equal to 0', (done) => {
+    expect(() => setServerConfiguration({
+      serverURL: 'http://localhost:8378/1',
+      appId: 'test',
+      appName: 'unused',
+      javascriptKey: 'test',
+      masterKey: 'test',
+      sessionLength: '-33'
+    })).toThrow('Session length must be a value greater than 0.');
+
+    expect(() => setServerConfiguration({
+      serverURL: 'http://localhost:8378/1',
+      appId: 'test',
+      appName: 'unused',
+      javascriptKey: 'test',
+      masterKey: 'test',
+      sessionLength: '0'
+    })).toThrow('Session length must be a value greater than 0.');
+    done();
+  })
 });