Fix handling notification/request parameters that is not used for class fields name from the server

[tom-tan]

Sorry, I accidentally send incomplete request...
I will update it to make this request complete.

Description (e.g. "Related to ...", etc.)

This request is to handle message fields that is not valid as python class fields (such as __dummy__) from the client.

For example, Eglot, which is a LSP client for emacs send initialized notification with __dummy__ parameter.

Currently language servers that are based on pygls fail with the following exception message:

Process EGLOT (v1/cwl-mode) stderr finished
Traceback (most recent call last):
  File "/Users/tanjo/repos/cwl-language-server/cwl_language_server/main.py", line 42, in <module>
    server.start_io()
  File "/Users/tanjo/.pyenv/versions/3.6.5/lib/python3.6/site-packages/pygls/server.py", line 164, in start_io
    self.lsp.data_received))
  File "/Users/tanjo/.pyenv/versions/3.6.5/lib/python3.6/asyncio/base_events.py", line 468, in run_until_complete
    return future.result()
  File "/Users/tanjo/.pyenv/versions/3.6.5/lib/python3.6/site-packages/pygls/server.py", line 65, in aio_readline
    proxy(body)
  File "/Users/tanjo/.pyenv/versions/3.6.5/lib/python3.6/site-packages/pygls/protocol.py", line 405, in data_received
    object_hook=deserialize_message))
  File "/Users/tanjo/.pyenv/versions/3.6.5/lib/python3.6/json/__init__.py", line 367, in loads
    return cls(**kw).decode(s)
  File "/Users/tanjo/.pyenv/versions/3.6.5/lib/python3.6/json/decoder.py", line 339, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/Users/tanjo/.pyenv/versions/3.6.5/lib/python3.6/json/decoder.py", line 355, in raw_decode
    obj, end = self.scan_once(s, idx)
  File "/Users/tanjo/.pyenv/versions/3.6.5/lib/python3.6/site-packages/pygls/protocol.py", line 86, in deserialize_message
    data.keys())(*data.values())
  File "/Users/tanjo/.pyenv/versions/3.6.5/lib/python3.6/collections/__init__.py", line 409, in namedtuple
    '%r' % name)
ValueError: Field names cannot start with an underscore: '__dummy__'

The reason is that __dummy__ that is sent from Eglot is not valid identifier for the field name in python.
By using rename=True for namedtuple, such fields are replaced with _1, _2 and so on.
Their names are very annoying but is better than server terminations.

Note: cwl_language_server is our project for a language server for Common Workflow Language.
We currently use pylspclient but we start trying pygls as an alternative.

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
  • This request does not add any functions.
• Standalone docs have been updated accordingly
  • This request does not contain the update that needs the doc update.
• CONTRIBUTORS.md was updated, as appropriate
• Changelog has been updated, as needed (see CHANGELOG.md)
  • No notable changes

[danixeee]

@tom-tan Good catch and thank you for your contribution!

Maybe we should consider adding multiple clients for our json-extension example, because we are testing it just with VS Code.

P.S. Check-boxes in Code review checklist from the PR template should be checked by a code reviewer, so I will uncheck them.

[danixeee]

@tom-tan Can you please remove the line break? Line 85. won't be longer than 79 chars.

[tom-tan]

Done!

[danixeee]

If you feel comfortable, it would be great to add some tests for deserialize_message to test_protocol.py. No worries if you can't, just let me know, so I can merge the PR otherwise.

[tom-tan]

Thank you for your suggestion.

I will update the code and add a test for it.

[tom-tan]

Done!

I added a test for deserialized_message.
I used an actual input for deserializing rather than deserializing '{ "__dummy__": true }' directly.
IMO it is easier to understand what this test wants to check.

[danixeee]

Looks go to me.

@tom-tan Can you please update the CHANGELOG file? We decided that every notable changes should be added to the CHANGELOG, and I think that this PR's change should be recorded there.

To make it easier for you, here is how it should look:

# Changelog

All notable changes to this project will be documented in this file.

The format is based on [Keep a Changelog][keepachangelog],
and this project adheres to [Semantic Versioning][semver].

## [Unreleased]

### Changed

- Your message here ([#56])

[#56]: https://github.com/openlawlibrary/pygls/pull/56

## [0.7.4] - 03/21/2019

### Added

- Add Pull Request template ([#54])

EDIT: Corrected PR link in example changelog.

[tom-tan]

Done!
I updated CHANGELOG.

[danixeee]

@tom-tan Looks go to me, will merge it soon.

Thank you again for your contribution! Let us know about your experience with pygls and if you have any language server that is based on it, feel free to update Implementations document. :)

[danixeee]

In Python, this is True for every valid first argument.