Make customer.io happy with the Content-Type (#5)

Fixes:

```
Traceback (most recent call last):
  File "/usr/local/lib/python3.10/site-packages/async_customerio/client_base.py", line 79, in send_request
    raise AsyncCustomerIOError(f"{result_status}: {url} {json_payload} {raw_cio_response.text}")
async_customerio.errors.AsyncCustomerIOError: 400: https://api-eu.customer.io/v1/send/email {...} {
  "meta": {
    "error": "Non-JSON content type application/json; UTF-8 in request header."
  }
}

During handling of the above exception, another exception occurred:
Traceback (most recent call last):
  File "/usr/local/lib/python3.10/asyncio/tasks.py", line 445, in wait_for
    return fut.result()
  File "/app/worker/jobs.py", line 18, in send_email
    return await customerio.send_email(request)
  File "/usr/local/lib/python3.10/site-packages/async_customerio/api.py", line 136, in send_email
    return await self.send_request(
  File "/usr/local/lib/python3.10/site-packages/async_customerio/client_base.py", line 83, in send_request
    raise AsyncCustomerIOError(
async_customerio.errors.AsyncCustomerIOError: Failed to receive valid response after 3 retries.
Check system status at http://status.customer.io.
Last caught exception -- <class 'async_customerio.errors.AsyncCustomerIOError'>: 400: https://api-eu.customer.io/v1/send/email {...} {
  "meta": {
    "error": "Non-JSON content type application/json; UTF-8 in request header."
  }
}
```

CHANGES.md

@@ -1,5 +1,9 @@
 # Changelog
 
+## 0.4.1
+
+- The ``Content-Type`` we use has been updated so that Customer.io is happy when we try to send emails.
+
 ## 0.4.0
 
 - Class ``AsyncCustomerIO`` updated with docstrings.

async_customerio/client_base.py

@@ -32,7 +32,7 @@ def _prepare_headers(self):
         """Prepare HTTP headers that will be used to request CustomerIO."""
         logging.debug("Preparing HTTP headers for all the subsequent requests")
         return {
-            "Content-Type": "application/json; UTF-8",
+            "Content-Type": "application/json",
             "X-Request-Id": self._get_request_id(),
             "User-Agent": "async-customerio/{0}".format(pkg_resources.get_distribution("async-customerio").version),
         }

pyproject.toml

@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "async-customerio"
-version = "0.4.0"
+version = "0.4.1"
 description = "Async CustomerIO Client - a Python client to interact with CustomerIO in an async fashion."
 license = "MIT"
 authors = [