Check if headers content-type is already set to avoid overwrite of pr…

[alessandrovisentini]

…eviously setted content-type in constructBodyParams method

When sending web conversions API to twitter using the post method of the library Twitter returns a 500 http error.

According to this twitter post to avoid the error the header 'content-type' : 'application/json;charset=UTF-8' must be used.

Even if the headers 'content-type' is setted in the post() method args field, the 'content-type' is then overwritten by the constructBodyParams() method. The solution could be using the forceBodyMode :'json' in the post() method but in doing so the library converts the entire body to a string by using JSON.stringify() and the "stringed" body is not valid and the Twitter API rejects it.

The solution is to accept the forced content-type from the post args field and not overwrite it.

[PLhery]

Makes sense, thank you for your contribution!

[PLhery]

Sorry I forgot to create the new version, now available in 1.12.10 :)