Bug/allow header to contain http verb keys #1252 (#1258)

* Failing test for #1252

* Only delete header keys that match an HTTP verb if the value is a non-string

Co-authored-by: David Ko <david.ko@pvtmethod.com>
Co-authored-by: Jay <jasonsaayman@gmail.com>

test/specs/headers.spec.js

@@ -86,4 +86,17 @@ describe('headers', function () {
       done();
     });
   });
+
+  describe('when a header key matches an HTTP verb', function() {
+    it('preserves the header value', function(done) {
+      axios.post('/foo', null, { 'headers': { 'delete': 'test' } });
+
+      getAjaxRequest().then(function (request) {
+        var headerValue = request.requestHeaders['delete'];
+
+        expect(headerValue).toEqual('test');
+        done();
+      });
+    });
+  });
 });