Allow ISO8859-1 (non UTF-8) .properties files.

[cmaglie]

Please check if the PR fulfills these requirements

See how to contribute

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

What kind of change does this PR introduce?

Previously .properties files (like library.properties or platform.txt) were considered always as UTF8 encoded. In some cases, this assumption leads to unwanted behavior.

Now the properties files are considered UTF8 if the files is a valid UTF8, otherwise, it is considered ISO8859-1.

What is the current behavior?

See #1907 for more details.

Library Manager doesn't show libraries. Output console shows
[INFO] ERROR: 2019/10/21 21:46:10 grpc: server failed to encode response: rpc error: code = Internal desc = grpc: error while marshaling: proto: field "cc.arduino.cli.commands.Library.Maintainer" contains invalid UTF-8

$ arduino-cli lib list "WiFi Link" --format json | jq .[].library.maintainer
"Juraj Andr�ssy <[email protected]>"

What is the new behavior?

The non-UTF8 file is handled correctly.

$ arduino-cli lib list "WiFi Link" --format json | jq .[].library.maintainer
"Juraj Andrássy <[email protected]>"

Does this PR introduce a breaking change, and is titled accordingly?

No

Other information

Fix #1907