Fix multipart to set its header even when other headers are provided without overwriting provided content type

[meuble]

In regards to issues opened about Content Type issue with multipart requests, also mentioned here in comments by @jirutka, there was a fix implemented.

I think this fix is incorrect as it prevent any Content Type customisation: if the request is multipart, it is always overwritten.
I propose a fix here that allow for Content Type customisation, but also makes sure that if we have custom headers but no Content Type, the content type is set to multipart/form-data.

[meuble]

We may want to include @TheSmartnik in the discussion to assess the viability of this pull request.

[TheSmartnik]

@meuble I've looked at the issue you've mentioned. Thanks for the pr, but It seems the problem in the issue is in this line. I simply hardcoded octet-stream, without detecting content-type of the file

[meuble]

@TheSmartnik That might be another issue.
The fact that we can't customise the Content Type header for requests with attachment remains, unless with the fix in my pr.

I don't claim to fix issue #595 here, but as it was a related issue, I though better to mention it.

[TheSmartnik]

@meuble To be honest, I can't think of any case where one would need to send a file content with any different content-type. Could give an example, please?

[meuble]

Yes.
Please refer to the following documentation http://rasa.com/docs/nlu/master/http/
As they specify:

The request should always be sent as application/x-yml regardless of wether you use json or md for the data format. Do not send json as application/json for example.

To do so, the HTTParty code I should write to query this API is the following

response = HTTParty.post("#{BASE_URL}/train", 
  body: {:file => File.open("intent_utterances.md")}, # Yes, it's a markdown file, here. Not a Yaml...
  headers: {'Content-Type' => "application/x-yml"}
)

In the current state of HTTParty, the Content Type that is sent to the API is multipart/form-data and I get an error. After my PR, the Content Type is application/x-yml and every thing is fine.
I agree that HTTParty should extract the content type from the file, if none is provided.
But even if HTTParty does it, I still want to have the freedom to specify a custom Content Type.

[TheSmartnik]

@meuble I see. Thanks!

You don't need multipart in this case. Following documentation, what you need to do is to send file directly. With httparty, you would do so, by streaming content of a file. Like so

HTTParty.put(
  'http://localhost:5000/train',
  body_stream: File.open('sample_configs/config_train_server_md.yml', 'r'),
  headers: { 'Content-Type' => 'application/x-yml', 'Transfer-Encoding' => 'chunked' }
)

This should produce expected payload.

---

Uploading files and uploading files through multipart is two different things. If we encode body params as multipart, but change content-type to anything else. Server won't be able to decode it

[meuble]

Ok, I wasn't aware of body_stream as a way to add files to HTTP requests.
I close the pull request. Sorry for the inconvenience...