Update README.md to explain custom file use case

Adding documentation to indicate a use case for custom file values and options in multipart form data requests. The added sentence may have saved my team a few hours.

Use case:
- A user wants to upload a file to your service, and you in turn want to upload the file to an external service.
- Constraint: you don't want the file to be written to disk in your service.
- Because you're uploading a file from a stream, you need to add custom file-related information manually in your request to the external service.
Upgrade `caseless` to 0.8.0

Fixes #1291.
Merge pull request #1292 from mmalecki/1291-upgrade-caseless

Upgrade `caseless` to 0.8.1
fix(proxy): no-proxy false positive

If one of the no-proxy item is exactly one character longer than the
request URL, it was considered as a valid match for no-proxy case
detection.

This patch handle the special (-1) value.
Merge pull request #1295 from oliamb/fix-issue-1294

fix(proxy): no-proxy false positive
2.49.0
Update changelog
2.49.1
Merge pull request #1288 from cliffcrosland/patch-1

Update README.md to explain custom file use case
Enable non-object JSON bodies

Updated JSON requests so they can contain bodies that are not objects. For avoid unexpected surprises, JSON `null` values are not sent.
Reverted null body behavior
More JSON request tests

Added test to ensure each type of JSON root object can be sent in a request
Updated test to verify JSON responses
Modifying JSON request test

Modified JSON request tests so the same value sent in the request
is echoed in the test server JSON response.