client: respond to KEEPALIVE(RESPOND) frame

[yuriykulikov]

Client sends KEEPALIVE(EMPTY) in this case.

Motivation and Context

This change is required for the rsocket-cpp client to be able to
work with current rsocket-java and rsocket-kotlin servers, which
do send KEEPALIVE(RESPOND) to the client.

How Has This Been Tested

Tested with rsocket-cpp, rsocket-kotlin and rsocket-java servers.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.