Refactored rendering sequence diagrams

Fixed default config clobbering issues
Updated/corrected sequenceDiagram.spec to set the config using mermaidAPI
Enabled freeze on mermaidAPI to protect defaultConfig

src/mermaidAPI.spec.js

@@ -1,10 +1,12 @@
 /* eslint-env jasmine */
 import mermaidAPI from './mermaidAPI';
+import { assignWithDepth } from './utils';
 
 describe('when using mermaidAPI and ', function() {
   describe('doing initialize ', function() {
     beforeEach(function() {
       document.body.innerHTML = '';
+      mermaidAPI.reset();
     });
 
     it('should copy a literal into the configuration', function() {
@@ -38,6 +40,52 @@ describe('when using mermaidAPI and ', function() {
       expect(config.testObject.test2).toBe(false);
       expect(config.testObject.test3).toBe(true);
     });
+    it('should reset mermaid config to global defaults', function() {
+      let config = {
+        logLevel: 0
+      };
+      mermaidAPI.initialize(config);
+      expect(mermaidAPI.getConfig().logLevel).toBe(0);
+      mermaidAPI.reset();
+      expect(mermaidAPI.getConfig()).toEqual(mermaidAPI.defaultConfig);
+    });
+    it('should prevent clobbering global defaults (direct)', function() {
+      let config = assignWithDepth({}, mermaidAPI.defaultConfig);
+      assignWithDepth(config, { logLevel: 0 });
+
+      let error = { message: '' };
+      try {
+        mermaidAPI['defaultConfig'] = config;
+      } catch(e) {
+        error = e;
+      }
+      expect(error.message).toBe('Cannot assign to read only property \'defaultConfig\' of object \'#<Object>\'');
+      expect(mermaidAPI.defaultConfig['logLevel']).toBe(5);
+    });
+    it('should prevent changes to global defaults (direct)', function() {
+      let error = { message: '' };
+      try {
+        mermaidAPI.defaultConfig.logLevel = 0;
+      } catch(e) {
+        error = e;
+      }
+      expect(error.message).toBe('Cannot assign to read only property \'logLevel\' of object \'#<Object>\'');
+      expect(mermaidAPI.defaultConfig['logLevel']).toBe(5);
+    });
+    it('should prevent sneaky changes to global defaults (assignWithDepth)', function() {
+      let config = {
+        logLevel: 0
+      };
+      let error = { message: '' };
+      try {
+        assignWithDepth(mermaidAPI.defaultConfig, config);
+      } catch(e) {
+        error = e;
+      }
+      expect(error.message).toBe('Cannot assign to read only property \'logLevel\' of object \'#<Object>\'');
+      expect(mermaidAPI.defaultConfig['logLevel']).toBe(5);
+    });
+
   });
   describe('checking validity of input ', function() {
     it('it should throw for an invalid definiton', function() {