OTLP HTTP Receiver Endpoints Give 404

[blakeroberts-wk]

Describe the bug
All the otlp http receiver endpoints give 404s with example local config.

Steps to reproduce

# start the collector running default local config (no modifications)
$ make run

# try to curl the traces endpoint
$ curl -X POST 0.0.0.0:55681/v1/traces
404 page not found

What did you expect to see?
I tested both v0.27.0 and v0.28.0

$ curl -X POST 0.0.0.0:55681/v1/traces
{

}

What did you see instead?
This is what you see starting with v0.29.0

404 page not found

What version did you use?
Version: v0.29.0

What config did you use?
Config: https://github.com/open-telemetry/opentelemetry-collector/blob/main/examples/local/otel-config.yaml

Environment
OS: macOS Big Sur version 11.2.3
Compiler(if manually compiled): go version go1.16.5 darwin/amd64

Additional context
I found this while upgrading a custom build I'm working on. I have a stable build using v0.27.0, but cannot upgrade to v0.29.0 due to this issue.

[bogdandrutu]

Duplicate, same as #3540

You need to include the "Content-Type" if you do a curl request.