[1.x] Avoid `application/x-protobuf` content type for Base64 client requests

[dmytro-kashcheiev]

Replace client request Content-Type from application/x-protobuf to something custom.

This issue applies to both web and dart repositories.

The default implementation of Protobuf controllers for some web servers (for example Micronaut) expects bytes when they face Content-Type:application/x-protobuf instead they receive Base64 encoded body that leads to additional configuration issues.
Probably some other Content-Type should be used.

[armiol]

Addressed for web via #194.