fix(headers): fixed common Content-Type header merging; (#5832)

test/specs/defaults.spec.js

@@ -151,12 +151,12 @@ describe('defaults', function () {
 
     getAjaxRequest().then(function (request) {
       expect(request.requestHeaders).toEqual(
-        utils.merge(defaults.headers.common, defaults.headers.get, {
+        AxiosHeaders.concat(defaults.headers.common, defaults.headers.get, {
           'X-COMMON-HEADER': 'commonHeaderValue',
           'X-GET-HEADER': 'getHeaderValue',
           'X-FOO-HEADER': 'fooHeaderValue',
           'X-BAR-HEADER': 'barHeaderValue'
-        })
+        }).toJSON()
       );
       done();
     });