[EXPORTER] Change OTLP HTTP content_type default to binary

[marcalff]

The OTLP HTTP exporter uses JSON by default.

According to the spec, the export format by default should be BINARY instead.

https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/protocol/exporter.md#specify-protocol

If no configuration is provided the default transport SHOULD be http/protobuf unless SDKs have good reasons to choose grpc as the default (e.g. for backward compatibility reasons when grpc was already the default in a stable SDK release).

Issue found by kkuehl in PR #2480