Fix typo Content-Type header in API client

[javlor]

Fix typo in the client header that could potentially return HTTP error 415 (unsupported media type). The correct spelling is "Content-Type". See Content-Type header.

[iantsednv]

@javlor, this is actually slightly more complicated than I thought. As summarized by review agent:

Background

This fix was reported as necessary to resolve HTTP 415 (Unsupported Media Type) errors in Google Colab. The root cause is real, but the fix is incorrect and introduces a different contract violation that will behave unpredictably across environments.

Why Google Colab returned 415

Google Colab routes outbound traffic through a Google-managed network proxy. With the original "ContentType": "application/json" (no hyphen), the outgoing POST included two relevant headers:

• ContentType: application/json — a malformed, unrecognised header
• Content-Type: multipart/form-data; boundary=... — correctly auto-set by requests

The Colab proxy, on encountering the malformed ContentType header alongside a valid Content-Type, appears to have dropped or rewritten the Content-Type before forwarding the request. The server received no valid Content-Type → 415.

Why the proposed fix works in Colab but is wrong

requests sets Content-Type in prepare_body only if it is not already present in the headers (checked via CaseInsensitiveDict):

# from requests/models.py
if content_type and ("content-type" not in self.headers):
    self.headers["Content-Type"] = content_type

By supplying "Content-Type": "application/json" in the shared headers dict, the caller's value wins and requests never overwrites it — even when building a multipart/form-data body with files attached. The server now receives:

• Content-Type: application/json — declared by our code
• Body: multipart-encoded binary data with a boundary

This is a broken HTTP request. It works in Colab only because the SolarFarmer API server is lenient and parses the multipart body regardless of the declared Content-Type. Any stricter server, reverse proxy, or future API version will reject it.

Correct fix

Remove Content-Type from the shared headers dict entirely. requests handles it correctly when left alone:

• GET requests: no body, no Content-Type needed
• POST with files: auto-set to multipart/form-data; boundary=<boundary>
• POST without files: auto-set to application/x-www-form-urlencoded

# solarfarmer/api.py — _make_request()
headers = {
    "Authorization": f"Bearer {key}",
    # Content-Type intentionally omitted — requests sets this automatically
    # based on whether files are present (multipart) or not (form-encoded).
    # Forcing it here overrides the boundary parameter and breaks multipart uploads.
    "User-Agent": "solarfarmer-api-sdk/" + __version__,
}

[iantsednv]

See my comment

[javlor]

@ollymaunder-dnv what do you think of the suggestion on Ian's agent to completely remove the Content-Type header?