Move HttpHeadersImpl class to internal and Handle error when websocket service is not running

[daneshk]

Purpose

This is an update PR to transport upgrade. This PR moves HeadersImpl class to internal and Handler error when websocket error is not started.
Resolves #503 , update netty-transport.yaml in stockquote sample.

Goals

This is an update PR to transport upgrade. This PR moves HeadersImpl class to internal and Handler error when websocket error is not started.

Approach

N/A

User stories

This is bind with transport version upgrade.

Release note

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Documentation

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Training

N/A

Certification

N/A

Marketing

N/A

Automation tests

N/A

Security checks

• Followed secure coding standards in http://wso2.com/technical-reports/wso2-secure-engineering-guidelines? yes
• Ran FindSecurityBugs plugin and verified report? yes
• Confirmed that this PR doesn't commit any keys, passwords, tokens, usernames, or other secrets? yes

Samples

Change existing sample.

Related PRs

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Migrations (if applicable)

N/A

Test environment

Linux environment. JDK 8

Learning

Websocket implementation.