implement adding Content-Length header

- derived from PR f#22

Author: urkle

graphql.js

@@ -116,6 +116,10 @@
       }
     }
 
+    if (cleaned.body) {
+      headers['Content-Length'] = cleaned.body.length;
+    }
+
     __doRequest(
       method,
       cleaned.url,

test/test.js

@@ -211,6 +211,13 @@ fragment auth_error on Error {messages}`;
                 expect(xhr.open).toHaveBeenCalledWith(method, expect.stringMatching(url), true)
                 expect(xhr.open).toHaveBeenCalledWith(method, expect.stringMatching(/\?query=.+&variables=/), true)
             });
+
+            it('does not send the content-length header', () => {
+                let xhr = mockXHR(200, {});
+                xhr.send = jest.fn();
+                fetchPost({id: 123});
+                expect(xhr.setRequestHeader).not.toHaveBeenCalledWith('Content-Length', expect.anything);
+            });
         });
 
         describe('when executing the queries normally', () => {
@@ -221,6 +228,13 @@ fragment auth_error on Error {messages}`;
                 expect(xhr.send).toHaveBeenCalled();
             });
 
+            it('sends the content-length header', () => {
+                let xhr = mockXHR(200, {});
+                xhr.send = jest.fn();
+                fetchPost({id: 123});
+                expect(xhr.setRequestHeader).toHaveBeenCalledWith('Content-Length', 99);
+            });
+
             it('resolves the response in the promise', () => {
                 let data = {post: {id: 123, title: 'title', text: 'text'}};
                 mockXHR(200, data);