Update the content type for post calls

[sunank200]

Currently, the POST request does not work properly in _do_api_call as the Content-Type HTTP header should be set for PUT and POST requests.

Here by default the content type is not set for POST which results in the 415 error like the following:

requests.exceptions.HTTPError: 415 Client Error: Unsupported Media Type for url: https://api.fivetran.com/v1/groups/

This is done as part of PATCH calls as https://github.com/fivetran/airflow-provider-fivetran/blob/main/fivetran_provider/hooks/fivetran.py#L117
A similar change is required for POST calls

References:
According to the RFC 7231 section 3.1.5.5:

A sender that generates a message containing a payload body SHOULD generate a Content-Type header field in that message unless the intended media type of the enclosed representation is unknown to the sender. If a Content-Type header field is not present, the recipient MAY either assume a media type of "application/octet-stream" ([RFC2046], Section 4.5.1) or examine the data to determine its type.

It means that the Content-Type HTTP header should be set for PUT and POST requests.

• Add the following line after: https://github.com/fivetran/airflow-provider-fivetran/blob/main/fivetran_provider/hooks/fivetran.py#L114

 headers.update({"Content-Type": "application/json;version=2"})

closes: #73

[utkarsharma2]

@sunank200 how did we verify this is working post the changes?

[thinhnd2104]

This is importance when customizing the hook and operator so we need to merge it ASAP. Thanks!!