Add extra hyphens

[LuisTejedaS]

From the issue:

By the specification for the Content-Type: multipart/form-data we should specify boundary marker. Once this marker is used in the body, it should contain two additional dashes in the beginning of the marker line.

We tried to generate the HTTP Code snippet and we can see that the dashes are not automatically added to the body fields and the last marker should be finished with two dashes, which is also not the part of the snippet code.

Expected result:

POST / HTTP/1.1
Host: www.google.com
Content-Length: 129
Content-Type: multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW

------WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="foo"

bar
------WebKitFormBoundary7MA4YWxkTrZu0gW--

Actual result:

POST / HTTP/1.1
Host: www.google.com
Content-Length: 124
Content-Type: multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW

----WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="foo"

bar
----WebKitFormBoundary7MA4YWxkTrZu0gW

Solution
Add the double hyphen at the beginning of the boundaries mark and the double hyphen at the end of the last boundary and

fixes: postmanlabs/postman-app-support#11084

[erfemega]

Looks good!

[LuisTejedaS]

@umeshp7 could we merge this already? or there is something else to do?