Fixed parsing bool value from query string

[Teclor]

When boolean value comes from query it is represented as 'true' or 'false string. However, it is not parsed until it is not a key for a map field. As a result, error message is thrown when boolean values are passed in GET requests (e.g. when some rest service is mapped to grpc one).
My suggestion: as long as field is defined as boolean, let's accept string 'true' or 'false' values too.

[google-cla]

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[anandolee]

@Teclor Thank you for this PR, but unfortunately we can't review it until you sign the CLA.

[Teclor]

@Teclor Thank you for this PR, but unfortunately we can't review it until you sign the CLA.

Hello, I signed CLA the day I made this MR, then I restarted check and now you can see it succeeded 3 days ago.
Here it is:
https://github.com/protocolbuffers/protobuf/pull/19028/checks

Also it looks passed in the check list here in MR

[Teclor]

@anandolee could you remove the "cla: no" tag from pull request, please? Or do I have to proceed some extra steps?