Ability to set the Content-Type header

[awwa]

• This pull request is intended to add an ability to set the Content-Type header, the python library as well.
• The trigger of this modify is making a request with except for "application/json" content-type which will be used in inbound parse projects.
• Also small fix for README example
• I will send CLA soon.

[awwa]

I sent the CLA.

[awwa]

I made additional fixes.

• It is difficult to test the final request parameter before the make_request. My approach for this is changing request local variable to Client member and add to attr_reader. This change makes me to be able to test it for several patterns.
  • test_build_request_post_empty_content_type
  • test_build_request_get_application_json
  • test_build_request_post_empty_body
  • test_build_request_post_multipart
• It seems the some assert_equal params (actual and expect) are the opposite.

[thinkingserious]

Hello @awwa,

Thanks again for the PR!

We want to show our appreciation by sending you some swag. Could you please fill out this form so we can send it to you? Thanks!

Team SendGrid DX